There are nearly a thousand caverns from Pennsylvania to Alabama. Hundreds of caves honeycomb the Ozark Mountains of Missouri and Arkansas, the central Texas plateau, and the mountains of the West. Some feature lights, elevators, stairways, and restaurants. Mammoth Cave, Carlsbad Caverns, and others have been added to the national park system. These noted caves are carved in limestone by the action of water.
Water absorbs a tiny amount of carbon dioxide and changes it to carbonic acid.
This acid dissolves limestone readily. In time all kinds of rocks develop cracks, and surface water works down into the cracks. When the rock is limestone, the water and acid dissolve it, slowly enlarging the cracks. If a limestone deposit has drainage deep underground, the water seeps in faster. Eventually the enlarged cracks becomes a hollow chamber, or a cave.