"subterranean" in the adjective sense
1. subterranean, subterraneous
being or operating under the surface of the earth
"subterranean passages"
"a subsurface flow of water"
2. 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
Source: WordNet® (An amazing lexical database of English)
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