Open English Wordnet
Adjectives
(s)subterranean, subterraneousbeing or operating under the surface of the earth“subterranean passages”
(s)subterranean, subterraneous, ulteriorlying 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)