Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† hypŏgēus, a, um, adj., = ὑπόγαιος, underground: loca, Cael. Aur. Tard. 2, 37, 191.
Hence, subst.: hypŏgēum or hy-pŏgaeum, i, n., = ὑπόγειον (ὑπόγαιον), a vault or cellar under ground, Vitr. 6, 11; a burying-vault, sepulchre, Petr. 111, 2; Inscr. Grut. 1114, 3.