Synonyms for subterranean

Adjective

1. subterranean, subterraneous, subsurface (vs. overhead) (vs. surface)

usage: being or operating under the surface of the earth; "subterranean passages"; "a subsurface flow of water"

2. subterranean, subterraneous, ulterior, covert (vs. overt)

usage: 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