Definitions

1. adjective

relating to or inhabiting a region below the surface of the earth, especially a cave or a subterranean area

“The hypogean environment in the cave was home to a unique species of subterranean fish.”

2. noun

a region below the surface of the earth, especially a cave or a subterranean area

“The guide led us through the hypogean passage, pointing out interesting formations and features.”