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One crack in layered rock, widened into a passage in three stages Three blocks of rock stand side by side. Each block is solid stone above and solid stone below, marked with thin dark lines for the flat layers it was laid down in, and each is split right through the middle by a gap with a slightly ragged roof and floor. In the first block that gap is a hairline. In the second it has opened into a slot. In the third it is a broad opening tall enough to walk through, while the rock above it and below it sits exactly where it always was.

A Cave Is Dissolved, Not Dug

About 14 minutes

Stand in a cave passage and the obvious question is where all the rock went.

There is a lot of it missing. A passage tall enough to walk down, running for hundreds of metres, is thousands of tonnes of stone that used to be there and is not. And there is no pile of it anywhere. No heap of rubble at the entrance, no spoil dumped in a side chamber.

That is because nothing broke the rock and nothing carried it out. The rock left as a liquid. It went out dissolved in water, through the crack the water arrived by, one speck at a time, over an unimaginable stretch of time.

Rain is not pure water

Rain starts out clean and does not stay that way. Falling through the air it picks up a little carbon dioxide — the same gas you breathe out. Then it soaks into the soil, and soil is full of roots and burrowing things and rotting leaves, all of which give off far more carbon dioxide than the air does.

Water with carbon dioxide dissolved in it becomes a very weak acid, called carbonic acid.

It is nothing frightening. It is what fizzy water is: gas pushed into water under pressure, making it slightly acidic, which is the faint sharp taste. Ordinary rainwater is far weaker than that.

Limestone gives in to it

Weak as that acid is, there is one common rock that cannot ignore it.

Limestone is made of calcium carbonate — the shells and skeletons of countless sea creatures, settled on an ancient sea floor and squeezed into stone. It is a hard rock. You can build a cathedral out of it.

But carbonic acid turns calcium carbonate into something that dissolves in water, the way sugar dissolves in tea. It does not crumble. It does not shatter. It simply stops being solid and joins the water, and then it goes wherever the water goes.

Water following the cracks that were already in a block of limestone A block of limestone is drawn from the side. Faint horizontal lines run across it for the flat layers the rock was laid down in, and faint upright lines run down through it for the splits made later when the rock was bent and lifted. A band of hatched soil sits on top. One route through the block is picked out in bold: it starts at the surface, drops down one of the upright splits, turns and runs a long way sideways along one of the flat layers, then drops again down another split. Every part of that bold route lies exactly on top of a faint line that was already there. soil, and soil is full of carbon dioxide flat layers upright splits The water invents nothing. It widens what was already open.
A block of limestone already comes cracked, in flat layers and upright joints. Water can only travel where a gap already exists — so those are the gaps that grow.

The cave is a map of the cracks

Here is the part that explains why caves look the way they do.

Water cannot dissolve rock it never touches. At the start there is no cave and no passage — just solid limestone with cracks in it. Flat gaps between the layers where the old sea floor was laid down, and upright splits running through them from the ground being bent and heaved over millions of years.

So the water goes into a crack, widens it very slightly, and next year more water goes in because the crack is bigger. That is a race the other cracks lose. Once one route is wider than its neighbours it takes more of the flow, so it grows faster, so it takes even more.

A hairline crack in limestone opening into a walkable passage Six pictures appear one after another from left to right. Each shows the same slab of rock, drawn as solid stone above and solid stone below with pale lines across it for the layers it was laid down in, and split through the middle by a gap. In the first the gap is a hairline you could barely get a sheet of paper into. In each picture after that the gap is wider than in the one before, while the rock around it stays exactly where it was. By the sixth the gap is a broad passage. A number under each picture gives the thousands of years that have gone by. 0 40 80 120 160 200 One crack. The rock around it never moves. thousands of years Nothing was pushed aside. The missing rock left as a liquid.

Slide the thousands of years forward and watch one hairline crack turn into somewhere you could stand up.

The passage does not appear in a new place. It is the crack, all the way through, only wider. That is why cave passages run in straight-ish stretches, meet each other at angles, and often sit in flat levels one above another — they are following joints and bedding planes that were in the rock long before any water arrived.

A cave is not a hole somebody made in a rock. It is a picture of where that rock was already weak.

The rock comes back out

The last stretch of the story runs backwards.

Water dripping through a cave roof has been carrying dissolved limestone, held there by the carbon dioxide in it. When that drip reaches an open, air-filled chamber, some of the gas escapes into the air — and without the gas the water can no longer hold its load.

So a tiny ring of stone is left behind, exactly where the drop hung. Then another. Then another, for a very long time.

Downward from the ceiling those rings build a stalactite. Where the drops land, the same thing builds upward into a stalagmite. (The one on the ceiling has the c; the one on the ground has the g.) They often grow at something like a centimetre in a hundred years, so a column as thick as your arm has been dripping since before anyone in your country could write.

Where did the rock that used to fill a cave passage actually go?

Where to look without going underground

You do not need a cave. Limestone and marble are used for old buildings, steps and gravestones, and the same slow chemistry has been working on them since the day they were cut.

Find an old stone step and look at the middle of it. Some of that hollow is feet. Some of it is a hundred years of rain, doing to a doorstep exactly what it does to a mountain.