Hypogeum is a word that refers to an underground structure or a subterranean chamber. However, there are other synonyms that can be used to express the same concept. One possible synonym for hypogeum is "catacomb," which describes an underground tomb or burial chamber. Another similar word is "crypt," which refers to a room or chamber for burying the dead. A third synonym is "subterranean," which describes something situated or occurring beneath the earth's surface. Finally, "cavern" is another possibility, which refers to a large underground chamber or cave. These words can be used interchangeably to describe underground spaces.