subterranean, subterraneous -- (being or operating under the surface of the earth; "subterranean passages"; "a subsurface flow of water")
subterranean, subterraneous, ulterior -- (lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed); "subterranean motives for murder"; "looked too closely for an ulterior purpose in all knowledge"- Bertrand Russell)
(Sound)
