Most caves are made in a specific landscape known as karst. Gypsum rocks, limestone and dolomite are dissolved by acidic rain to form caves. It takes more than 100,000 years for most caves to become large enough to accommodate humans. Many caves exist in a system of interconnected underground pathways.
In addition to water erosion, caves are formed by pressure, chemical reactions, volcanic forces, microorganisms and tectonic movement. Caves that are created on the coasts by tidal movement are known as sea caves. Melting ice can create glacier caves.
Humans have used caves as shelter, art canvases, locations for religious rites and burials throughout history. Criminals, such as Jesse James have used caves as both a hideout and a place to store stolen goods.
There are more than 55,500 caves throughout the United States. The state of Missouri features more than 6,000 caves. Some of the most famous caves in the country include Carlsbad Caverns, Meramec Caverns, Mammoth Cave and Jewel Cave. Niagara Cave contains ancient fossils more than 400 million years old. Georgia is home to the deepest cave in the world, Voronya Cave. Hobbies that include cave exploration are known as spelunking, caving and potholing. Speleology is the study of caves.