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 often extend deep underground. The word "cave" can also refer to much smaller openings such as sea caves, rock shelters, and grottos.

It is estimated that the maximum depth of the cave cannot be more than 3,000 meters: deeper caves are crushed by the weight of overlying rock. For karst caves the maximum depth is determined on the basis of karstovaniya (the lower limit of karst marking processes, coinciding with the base sequences limestone), which may be lower than the basis erosion due to the presence of siphon channels.

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).