From WordNet (r) 3.0 (2006) [wn]:
cavern
n 1: any large dark enclosed space; "his eyes were dark caverns"
2: a large cave or a large chamber in a cave
v 1: hollow out as if making a cavern [syn: cavern, {cavern
out}]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Cavern \Cav"ern\, n. [L. caverna, fr. cavus hollow: cf. F.
caverne.]
A large, deep, hollow place in the earth; a large cave.
[1913 Webster]