specus

Specus, specus, fourth declension, and unusually loose about gender: masculine most often, but feminine and neuter are both attested in good authors, so an adjective agreeing with it can go either way. Nominative and genitive singular are identical, as they are throughout the fourth declension.

Antrum is the poetic word for the same hole in the hillside, with nymphs in it; specus is the plain one, and it stretches to any hollow at all, including a mine shaft or a covered drain. The root is the one behind specio, "look", the idea being an opening you can see into.