How Do Caves Form?

Step 1 of 5

Rain is very slightly acidic.

Falling through the air and soaking through soil, water picks up carbon dioxide. That makes a weak acid — far too weak to hurt you, strong enough to matter.

Step 2 of 5

Limestone dissolves in that acid.

Limestone is calcium carbonate, and weak acid takes it into solution. Most other rocks shrug it off, which is why big cave systems are almost always limestone.

Step 3 of 5

Water follows cracks, and the cracks grow.

Water seeps into existing joints. Every trickle takes away a little rock, widening the crack, which lets more water through — and it accelerates.

Step 4 of 5

Draining water leaves the chamber empty.

Once the water table drops, the dissolved-out passage is left full of air. That is the moment a flooded channel becomes a cave you can walk into.

Step 5 of 5

Then the drips build it back up.

Water dripping into the air loses its carbon dioxide and drops its dissolved limestone. That builds stalactites down and stalagmites up.

Stalactites often grow around a centimetre a century. A metre-long one has been dripping since the Roman empire.