Dictionary:

hypogeum

  (hī'pə-jē'əm)

1. A subterranean chamber of an ancient building.
2. An ancient subterranean burial chamber, such as a catacomb.

[Latin hypogēum, from Greek hupogeion, from neuter of hupogeios, underground. See hypogeal.]

In ancient architecture, any underground chamber or vault, esp. an underground burial chamber.