Hypogeum

Hypogeum or hypogaeum (plural hypogea) literally means "underground", from Greek hypo (under) and gaia (mother earth or goddess of earth). It usually refers to an underground temple or tomb.

The later Christians built similar underground shrines, crypts and tombs, which they called catacombs. But this was only a difference in name, rather than purpose and rituals, and archeological and historical research shows they were effectively the same. Werner Jacobsen wrote,