*hypogeum*

‖n.; pl. Hypogea [ L., fr. Gr. &unr_;, &unr_;, subterranean; &unr_; under + &unr_;, &unr_;, the earth. ] (Anc. Arch.) The subterraneous portion of a building, as in amphitheaters, for the service of the games; also, subterranean galleries, as the catacombs. [ 1913 Webster ]