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Caves and Karst – Landscapes Dissolved by Water · Talking Science

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Next time it rains, I want you to try something.
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Oh, what's that? Well, just stand at the window,
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watch a puddle form, and just track what happens
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to that water when it hits the ground and disappears.
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Right, because usually we just assume it's, you
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know, soaking harmlessly into the soil. Exactly.
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Watering the grass, feeding the trees. It's a
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very peaceful, mundane image, right? It is. But
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in certain landscapes scattered all over the
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world... that peaceful image is a complete illusion
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wait an illusion yeah that disappearing rainwater
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isn't just soaking in it is actively dissolving
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solid rock patiently over tens of thousands of
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years that seemingly harmless water is carving
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out vast hidden networks of caves and tunnels
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right beneath your feet welcome to the deep dive
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Today our mission is to completely change how
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you look at a rainy day. We are venturing into
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the hidden, completely silent world of what geologists
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call karst landscapes. It is a wild topic. It
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really is. We're going to explore massive underground
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architecture, ecosystems that operate like alien
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planets, and ancient time capsules, and all of
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it sculpted entirely by ordinary water. Okay,
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let's unpack this. What exactly is a karst landscape
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and how does water actually destroy solid rock?
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Well, to understand karst, we have to start with
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the rock itself. So karst landscapes form mostly
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in areas underlain by limestone. Okay, limestone.
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Yeah, and limestone is a sedimentary rock, meaning
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it was built up in layers over vast stretches
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of time. It's made primarily of calcium carbonate.
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Which comes from what exactly? It actually comes
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from the accumulated shells and skeletons of
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ancient marine creatures. They settled at the
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bottom of long vanished oceans. I mean, millions
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of years ago. So if you're standing in a karst
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landscape today, you're essentially walking on
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millions of years of compressed seashells. That
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is the reality of it. Yeah. And physically speaking,
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limestone is incredibly tough. It stands up very
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well against physical erosion. The wind blowing
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against it or water mechanically scouring it,
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that won't easily grind it down the way they
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might with softer rocks like sandstone. But it
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has a weakness. It has one massive vulnerability,
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and that is its chemistry. Calcium carbonate
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reacts very easily with weak acids. Which brings
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us back to the rain. But rain on its own isn't
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inherently an acid, is it? I mean, it's just
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water. Not initially, no. As rain falls through
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the atmosphere, it picks up a little bit of carbon
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dioxide. But the real chemical transformation
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happens when that water hits the dirt. The dirt.
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Yeah, the soil is absolutely packed with CO2.
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Plant roots, fungi, decaying organic matter,
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they are all constantly exhaling it. So when
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the rainwater percolates through that rich soil,
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it absorbs a massive dose of carbon dioxide.
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Oh, I see. And when that gas mixes with the water,
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it forms carbonic acid. Wait, carbonic acid,
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isn't that the exact same stuff that makes soft
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drinks fizzy? It is the exact same chemical,
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yes. That's crazy. Yeah. So rainwater is basically
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a very weak, flat soda quietly dissolving the
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ground beneath our feet. Pretty much. And think
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about the sheer scale of that. When this slightly
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acidic soda touches the limestone bedrock, it
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doesn't just mechanically push the rock out of
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the way. It eats it. Exactly. It chemically dissolves
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it. It is literally washing the rock away atom
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by atom, turning solid calcium carbonate into
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liquid dissolved minerals. And I imagine this
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isn't happening perfectly evenly across a flat
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surface, right? The water is going to find the
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easiest path down into the earth. Right. The
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water seeks out tiny cracks and fractures already
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present in the bedrock. These are microscopic
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fissures left over from when the rock formed,
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or maybe from ancient tectonic stress. So looking
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for the weak points. Always. Water naturally
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prefers the path of least resistance, so it funnels
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into these existing weaknesses. As it flows,
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it dissolves the rock along the edges of the
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crack, making it just... tiny bit wider. Which
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means the next time it rains, that slightly wider
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crack can hold even more water. Exactly. It's
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a relentless, self -reinforcing loop. Wider cracks
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allow more water to flow, and more water brings
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more carbonic acid. Which means faster dissolving.
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You got it. The passages that start out even
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a millimeter larger will keep growing disproportionately
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faster than the narrower cracks nearby. What's
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fascinating here is the sheer paradox of it.
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We often think of geological change as violent,
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right? Like volcanoes erupting, earthquakes splitting
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the ground, massive floods. Exactly. But here,
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a mild invisible chemical reaction achieves what
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violent mechanical erosion cannot. It just takes...
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deep time deep time meaning we're talking tens
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of thousands hundreds of thousands sometimes
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millions of years it's a process we can measure
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and understand perfectly but no single human
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will ever see a cave formed from beginning to
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end wow so we have this acidic water eating its
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way down through the fractures but eventually
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that water has to go somewhere gravity pulls
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it down until it hits a limit and i guess this
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brings us to how these actual underground rooms
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and tunnels take shape right because dissolution
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doesn't happen As the rainwater soaks down, it
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eventually reaches a boundary known as the water
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table. Okay, the water table. Below the water
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table, every single crack, pore, and fissure
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in the rock is completely and permanently saturated
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with groundwater. Geologists call this saturated
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area the free attic zone. So the water table
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is essentially the underground fill line. Everything
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below it is flooded. Spot on. And the area above
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that fill line, where the water is just temporarily
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trickling downward under the pool of gravity,
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that is called the Vados Zone. Vados Zone. Okay.
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Now, the most vigorous, aggressive cave development
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tends to happen right exactly at the water table
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boundary itself. Why is that? Because this is
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where the acidic water stops moving purely vertically
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and starts moving horizontally, carving out wide
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tunnels and expansive chambers. But over geological
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time, the water table doesn't just stay in one
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place, right? Over tens of thousands of years,
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sea levels drop or a river on the surface cuts
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a deeper valley into the landscape. Right. And
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that essentially pulls the underground water
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level down with it. So what happens to the cave?
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Well, when that regional water table drops, the
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older cave passages that formed right at that
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boundary get left high and dry. The water drains
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out of them entirely. Oh, wow. Yeah, and because
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they are no longer submerged in that flowing
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acidic river, they stop being actively enlarged.
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Meanwhile, lower down in the rock, at the new
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water table, a brand new horizontal passage begins
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to form. It's like an abandoned apartment building
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where only the basement has running water. And
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every floor above is just a fossil of where the
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plumbing used to be. That is a perfect way to
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put it. Over vast stretches of time, as the regional
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geology shifts, you get these incredibly complex,
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multi -story stacked cave systems. So cool. Each
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level of the cave is a structural fossil, showing
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you exactly where the water table used to sit
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deep in the past. And often, if you climb all
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the way down to the lowest, most active level
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of a cave today, you'll find a genuine, flowing
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underground river still doing the work of carving
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the basement. Here's where it gets really interesting.
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The water drains out of those upper apartments.
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But they don't just sit empty. No, they don't.
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Because they are now high and dry, filled with
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air, they become massive, empty canvases for
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a secondary process. The geology shifts from
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destroying rock to decorating it. And in the
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process... creates a geological hard drive of
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the Earth's history. Yeah, we enter the realm
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of speleothems, which is the scientific umbrella
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term for all those striking cave formations.
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The stalactites and stalagmites. Exactly. Picture
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our empty, dry cave passage. High above, water
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is still seeping down through the limestone roof.
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That water is carrying a heavy load of dissolved
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calcium carbonate. The minerals have dissolved
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out of the rock on its way down. Right. But the
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environment completely changes the second that
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drop of water breaches the ceiling and hits the
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open air of the cave chamber. Oh, so. Well, when
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that droplet hangs from the ceiling, a crucial
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chemical reversal happens. Remember how the water
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absorbed carbon dioxide in the soil to become
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acidic? Yeah, the whole flat soda thing. Right.
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Well, the air inside the open cave has much less
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CO2 than the soil above. So when the water droplet
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hits the cave air, some of that dissolved carbon
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dioxide escapes. Think of it like taking the
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cap off a shaken bottle of soda. It fizzes out.
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Exactly. The CO2 off -gases into the air. And
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if the CO2 leaves the water, the water becomes
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less acidic. You got it. And because it is less
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acidic, it can no longer hold all of that dissolved
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calcium carbonate in a liquid state. The chemistry
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forces its hand. So what does the water do with
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the minerals? Before the droplet even falls,
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it deposits a tiny microscopic trace of solid
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calcium carbonate right there on the ceiling.
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Just drop by drop, layer by layer, building over
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centuries. Exactly. These downward -pointing
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formations are called stalactites. And if that
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water droplet still has some dissolved minerals
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in it when it finally falls and hits the floor...
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Oh, the impact releases more CO2? Yes. Depositing
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more calcium carbonate there on the ground? This
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builds upward -growing formations called stalagmites.
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Given enough millennia, a stalactite and a stalagmite
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might even meet in the middle to form a massive
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solid column. I always get them mixed up, but
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for anyone trying to keep them straight in their
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head, the classic memory trick is perfect here.
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Stalactites hold tight to the ceiling, and stalagmites
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might one day reach it. That's the one. And if
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we connect this to the bigger picture... These
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speleothems are far more than just beautiful
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subterranean architecture. Right. You mentioned
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a geological hard drive earlier. Yeah. Because
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they grow in distinct measurable layers, very
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much like tree rings, they are actively trapping
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the chemistry of the environment at the exact
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moment that specific layer was formed. It's quite
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literally a climate diary written in stone. It
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really is. Scientists who study the climate of
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the deep past, paleoclimatologists, they can
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take a core sample. straight through the center
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of a stalagmite. Just drill right through it.
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Yep. By analyzing those microscopic layers, they
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look at variations in isotopes. Isotopes. Remind
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me what those are. Sure. Isotopes are basically
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just versions of a chemical element like oxygen
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that are minutely heavier or lighter because
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they have a slightly different number of neutrons
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in their nucleus. So what are molecules with
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heavier or lighter oxygen isotopes? Evaporate
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and fall as rain differently. depending on the
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global temperature. Exactly. By measuring the
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ratio of those isotopes trapped in a layer of
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cave rock, you know what the weather was like
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on the surface. 10 000 years ago yes down to
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a yearly or sometimes even seasonal resolution
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that is wild and to know exactly when that weather
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happened they rely on another mechanism as the
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water drips into the cave it carries trace microscopic
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amounts of uranium from the rocks above uranium
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like radioactive uranium yeah just tiny trace
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amounts which gets trapped in the stalagmite
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layers uranium naturally decays into other elements
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like thorium at a very predictable steady rate
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so it acts as a built and geological stopwatch.
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Precisely. By measuring the ratio of uranium
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to thorium in a specific layer, they get an incredibly
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accurate timestamp for that exact climate record.
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These cave records provide a totally independent
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cross -check against other climate archives,
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like polar ice cores, giving us a highly detailed
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picture of Earth's history. It's just staggering
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to think about. While these invisible layers
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are slowly building underground, keeping track
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of the weather, the void beneath is quietly altering
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the surface world we actually live on. Oh, absolutely.
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Let's talk about what happens when the ground
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gives way. Because karst landscapes look completely
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different on the surface. They do. The exposed
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limestone on the surface often has this jagged,
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pitted, almost sculptural quality from the constant...
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chemical weathering. But the most dramatic surface
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feature is undeniably the sinkhole. Which is
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exactly what it sounds like. But there are a
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couple of ways they form, aren't there? It's
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not always a sudden catastrophe. No, sometimes
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it's a very slow, gradual subsidence. As the
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limestone underneath slowly dissolves over centuries,
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the soil sitting on top just gently settles down
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to fill the newly created gap. So you just get
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a dip in the ground. Right. It creates a soft,
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bowl -shaped depression in a field. You might
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not even notice it happening over a human lifetime.
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But then they're the ones that make the evening
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news. The catastrophic collapses. Yeah. This
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comes down to simple physics and gravity. If
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an underground cavern grows large enough, the
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layer of rock or soil forming its roof gets increasingly
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thin. And eventually it just gives out. Eventually
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that roof can no longer support its own weight,
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especially if there's a heavy rainstorm that
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suddenly adds thousands of pounds of water weight
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to the soil above. Without warning, the roof
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caves in. Terrifying. That's when you get a steep
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-sided hole that can swallow a section of a highway
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or a building or a patch of paddock in a matter
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of seconds. It's a terrifying thought. And doesn't
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just swallow roads. It swallows entire rivers.
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Our source text talks about disappearing streams.
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It's like a river hits surface traffic and just
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decides to take the subway instead. That's a
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great analogy. You'll have a creek flowing along
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perfectly normally on the surface, and then it
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encounters a spot where the bedrock has dissolved
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into an open vertical passage, often called a
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swallowhole. And it just goes down. The entire
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stream just pours down into the ground continuing
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its journey entirely in the dark. It might run
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underground for miles before reappearing as a
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spring at the base of a valley. This is why karst
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landscapes, even in areas with heavy rainfall,
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often have surprisingly few permanent surface
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lakes or streams. The water simply doesn't stay
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on top. Because the hydrology is all happening
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in the basement. Exactly. It really highlights
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the constant tension in a karst landscape between
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the solid surface we blindly trust and the hollow,
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actively growing spaces beneath it. Now, when
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those rivers vanish underground, they don't just
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carry water, right? They carry organic debris,
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which brings us to the life that actually survives
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down in that plumbing. The ecosystems down there
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are incredible. Yeah, you might think these dark,
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isolated underground subways are just empty geological
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voids, but they actually host some of the strangest
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ecosystems on Earth. But wait, I'm stuck on something
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here. If the sun is the battery for almost all
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life on Earth, fueling the plants through photosynthesis,
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which in turn fuels the herbivores and carnivores,
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how is an entire food web surviving down there
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in total permanent darkness? It is one of the
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most brilliant biological workarounds in nature.
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Deep inside these caves, there is absolutely
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zero sunlight, which means zero photosynthesis.
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No plants can grow. So where does the energy
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come from? The fundamental nutrients have to
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be imported from the outside world. While some
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nutrients wash in with those disappearing streams,
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the real heroes of the deep cave ecosystem are
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bats. Ah, the biological conveyor belt. You got
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it. Many bat species roost in these deep cave
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systems by the millions during the day, treating
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it as a safe, climate -controlled shelter. Makes
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sense. At night, they fly out into the surrounding
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landscape, feeding on insects or fruit energy
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that ultimately came from the sun. Then they
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return to the deep cave to sleep, and they deposit
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their droppings, known as guano, deep inside
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the dark zone. And that guano becomes the absolute
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foundation of life in the dark. It is incredibly
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nutrient dense. Entire specialized food webs
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build up around these guano deposits. You have
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specialized mites, beetles, cockroaches, and
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fungi that exist nowhere else on Earth. Just
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living off bat poop. All surviving purely because
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bats are commuting back and forth, bringing the
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sun's energy down into the dark. That is wild.
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And the creatures that live down there permanently,
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the troglobites, are fascinating. Our source
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notes this phenomenon called convergent evolution,
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where completely unrelated species living in
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different isolated caves all over the world evolve
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the exact same strange traits because they are
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facing the exact same. intense environmental
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pressures. It's nature ruthlessly optimizing
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for an extreme environment. Think about it. Every
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physical biological trait requires calories to
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grow and maintain. If you live your entire life
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in permanent pitch black darkness, building and
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maintaining an eye is a catastrophic waste of
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biological energy. Because you literally can't
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use it. So over generations, many troglobites
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have lost their eyes entirely or just have tiny
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non -functional remnants covered by skin. Because
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if you're wasting energy on eyes you can't use,
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you're going to starve before you can reproduce.
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for color yeah producing pigment also costs energy
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On the surface, pigment provides camouflage from
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predators or protects against UV light from the
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sun. In a pitch black cave, neither of those
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threats exist. So they don't need it. Exactly.
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So troglobites often appear pale, white, or completely
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translucent. But they must have to compensate
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somehow to find that scarce food in the dark.
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Oh, they do. They evolve dramatically heightened
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senses of touch and smell. You'll see blind cave
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insects with incredibly elongated antennae or
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exceptionally long, spindly legs. to navigate
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their world entirely by feel. Just feeling their
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way around. And crucially, they have markedly
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slowed metabolisms. Food in a cave is incredibly
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scarce and unpredictable. If a disease hits the
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bat colony or they migrate, the food stops. Right,
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the conveyor belt shuts down. Exactly. If a troglobite
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can survive comfortably on almost no food for
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long stretches of time, it has a massive survival
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advantage. But that slow metabolism and reliance
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on a single nutrient source means the whole system
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is incredibly fragile. If human disturbance drives
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the bats away, the nutrient conveyor belt stops,
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and the whole underground food web quietly starves
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to death. It really is. It's a perfect example
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of nature making the absolute most out of a harsh
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environment, but balancing on an absolute knife's
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edge. The isolation that protects them also makes
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them incredibly vulnerable to any sudden change.
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To really visualize all of these incredible features,
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the deep time, the stacked architecture, the
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strange biology, we can look at some iconic karst
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sites right here in Australia. because they manifest
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in such wildly different ways depending on the
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local geology. Australia is a phenomenal showcase
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for this. It really is. Take the Genolan Caves
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in the Blue Mountains of New South Wales. These
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are among the oldest known open cave systems
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in the world. How old are we talking? We're talking
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hundreds of millions of years old. They are packed
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with stunning massive speleothems, and they still
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have active underground rivers carving away the
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basement levels today. And on the completely
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opposite end of the visual spectrum, you have
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the Nullarbor Plain. stretching across the South
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Australia and Western Australia border. The name
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Nullarbor literally means no trees in Latin.
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It's one of the largest single exposures of flat
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limestone karst on the entire planet. From the
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surface, it looks vast, flat, and completely
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desolate. But underneath. Beneath that arid surface
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is an extensive labyrinthine network of caves
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that hold underground lakes and hide some truly
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remarkable ancient animal fossils. Which brings
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us to perhaps the most amazing time capsule of
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all. The Narakort Caves in South Australia, which
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is actually a UNESCO World Heritage Site. Yes.
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Narakort is globally significant because of its
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specific type of cave entrances. Okay. Tell me
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about those. Well, over hundreds of thousands
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of years, there were these natural, steep pitfall
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entrances hidden in the karst landscape. Animals
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walking on the surface would occasionally fall
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in, dropping dozens of feet into the darkness,
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and they simply couldn't climb back out. Wow.
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It was a completely unintentional, naturally
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forming trap. Yeah. And because the interiors
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of these caves are so stable protected from the
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wind, the sun, and the elements, the remains
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of those animals were preserved perfectly. That
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is incredible. It became one of the richest,
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most pristine fossil deposits of Ice Age Australian
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megafauna anywhere on the continent. Paleontologists
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have found the complete skeletons of creatures
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like the marsupial lion. Oh, the thylacolio.
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Yes. Thylacolio, a massive mammalian predator
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with incredibly powerful jaws that roamed Australia
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thousands of years ago. alongside giant kangaroos
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and enormous wombats. It's a remarkably detailed
20:31
window into a thriving ecosystem that vanished
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long ago, perfectly preserved by the quiet darkness.
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So what does this all mean? We've got acid rainwater
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carving out stone. climate wreckers hanging from
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ceilings, blind translucent insects, and giant
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fossilized marsupial lions hidden under flat
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plains. A lot to take in. It is. We've followed
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the journey of a single drop of rain today, pulling
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carbon dioxide from the soil to become carbonic
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acid, finding a microscopic crack in the limestone,
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and joining the water table in the saturated
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phreatic zone to carve out massive chambers.
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And then watching the water drain away. Right.
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Leaving empty apartments where new drops leave
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behind calcium carbonate, recording the global
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temperature like a tree ring. We've seen how
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that growing underground void can suddenly swallow
21:17
a river or a road, and how the darkness it leaves
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behind feeds pale, blind creatures waiting for
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a bat to drop nutrients from the sunlit world
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above. This raises an important question about
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permanence, deep time, and our own place in this
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geological story. How so? Well, these karst spaces
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act as silent witnesses to history. It is an
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entire landscape being built in reverse, patiently
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carved away from the inside out, completely in
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silence. It's a beautiful way to put it. The
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same patient, quiet conditions that build a stalagmite
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drop by drop also preserve the delicate bones
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of megafauna that the turbulent surface world
21:53
would have ground to dust millennia ago. It's
21:56
true. Which leaves us with a fascinating thought.
21:58
Okay, lay it on me. If the pitfall traps at Narakort
22:01
perfectly preserved the giant Ice Age megafauna,
22:04
What artifacts or evidence of our modern human
22:07
era are currently falling into the dark, quiet
22:09
depths of today's sinkholes and swallowholes?
22:12
Oh, wow. What pieces of our daily lives are being
22:15
quietly encased in mud and calcium carbonate,
22:18
just waiting, perfectly preserved, for paleontologists
22:21
to find hundreds of thousands of years in the
22:23
future? That is a brilliant point. Yeah. Something
22:25
to think about the next time you watch the rain
22:27
disappear into the earth.