Step one: Rain seeps into the surface of the earth.
Step two: The gas, carbon dioxide (CO 2 ), is released by dying vegetation in the soil.
Step three: The rainwater absorbs the carbon dioxide ( CO 2 ).
Step four: A chemical reaction begins that makes the water into a weak carbonic acid .
Step five: The carbonic acid eats away at rock by dissolving the limestone.
Step six: A cave passage is formed.
Look what happened in Hidden Cave after a rainstorm, pretty easy to see how rain helped form this cave!
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If you'd like to see an animation of this, go to: