Lea Rawls con The Caves: Diving (Photo Book Book 166) (English Edition)A cave is a hollow place in the ground, specifically a natural space large enough for a human to enter. Caves form naturally by the weathering of rock and often extend deep underground. The word cave can also refer to much smaller openings such as sea caves, rock shelters, and grottos, though strictly speaking a cave is exogene, meaning it is deeper than its opening is wide, and a rock shelter is endogene. A cavern is a specific type of cave, naturally formed in soluble rock with the ability to grow speleothems.
Speleology is the science of exploration and study of all aspects of caves and the cave environment. Visiting or exploring caves for recreation may be called caving, potholing, or spelunking.
Caves can be classified in various other ways as well, including active vs. relict; active caves have water flowing through them, relict caves do not, though water may be retained in them. Types of active caves include inflow caves ("into which a stream sinks"), outflow caves ("from which a stream emerges"), and through caves ("traversed by a stream").
Solutional caves or karst caves are the most frequently occurring caves. Such caves form in rock that is soluble; most occur in limestone, but they can also form in other rocks including chalk, dolomite, marble, salt, and gypsum.
Caves formed at the same time as the surrounding rock are called primary caves.
Lava tubes are formed through volcanic activity and are the most common primary caves.
Sea caves are found along coasts around the world. A special case is littoral caves, which are formed by wave action in zones of weakness in sea cliffs.
Corrasional or erosional caves are those that form entirely by erosion by flowing streams carrying rocks and other sediments.
Glacier caves are formed by melting ice and flowing water within and under glaciers. The cavities are influenced by the very slow flow of the ice, which tends to collapse the caves again.
Fracture caves are formed when layers of more soluble minerals, such as gypsum, dissolve out from between layers of less soluble rock. These rocks fracture and collapse in blocks of stone.
Talus caves are formed by the openings among large boulders that have fallen down into a random heap, often at the bases of cliffs.
Anchialine caves are caves, usually coastal, containing a mixture of freshwater and saline water (usually sea water).