A natural cavity located underground or in the side of a hill or cliff, generally of a size to admit a human. Caves occur in all types of rocks and topographic situations. They may be formed by many different erosion processes. The most important are created by ground waters that dissolve the common soluble rocks—limestone, dolomite, gypsum, and salt. Limestone caves are the most frequent, longest, and deepest. Lava-tube caves, sea caves created by wave action, and caves caused by piping in unconsolidated rocks are the other important types. The science of caves is known as speleology.