Any naturally occurring void, cavity, recess, or system of interconnected passages which occurs beneath the surface of the earth or within a cliff or ledge and which is large enough to permit an individual to enter, whether or not the entrance is naturally formed or human-made. This includes any natural pit, sinkhole, or other feature which is an extension of the surface. (From the Federal Cave Resource Protection Act, 1988.)

any large dark enclosed space; "his eyes were dark caverns"

a large cave or a large chamber in a cave

hollow out as if making a cavern

a connected system of caves and passages

a natural environment

Synonym for cave, especially one that is very large

A cave of large or indefinite extent.