subterraneanadjective
US /ˌsʌbtəˈreɪniən/
Existing or taking place beneath the surface of the earth
“After having passed with tolerable ease through the subterranean passage, which, however, did not admit of their holding themselves erect, the two friends reached the further end of the corridor, into which the abbé’s cell opened; from that point the passage became much narrower, and barely permitted one to creep through on hands and knees.”
✓ Engvera
