Cave
As rain travels
through the soil, it gathers carbon dioxide. Water mixed with carbon dioxide
makes an acid which cracks through the rocks and forms a cave. There are
underwater caves too. Caves are underground chambers that take thousands of
years to form! Caves are very deep and some have a little water in them. Most
caves have an entrance on the surface of the earth. There are caves everywhere
that you can visit. It's hard to tell how old a cave is.