Hypogeum
Hypogeum , noun
[Latin, from Greek {not transcribed}, {not transcribed}, subterranean; {not transcribed} under + {not transcribed}, {not transcribed}, the earth.]
(Anc. Architecture) The subterraneous portion of a building, as in amphitheaters, for the service of the games; also, subterranean galleries, as the catacombs.