The Collaborative International Dictionary of English v.0.48:

Hypogeum \Hyp`o*ge"um\, n.; pl. Hypogea. [L., fr. Gr. ?, ?,
   subterranean; ? under + ?, ?, 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]