A cave or cavern is a natural underground space large enough for a human to enter. Caves form naturally by the weathering of rock and they often extend deep underground. The word "cave" can also refer to much smaller openings such as sea caves, rock shelters, and grottos.
Speleology is the science of exploration and study of all aspects of caves and the environment that surrounds the caves. Exploring a cave for recreation or science may be called caving, potholing, or, in Canada and the United States, spelunking (see caving).