Synonyms for Subterraneous (same or very similar meaning)
WordNet sense 1 (being or operating under the surface of the earth):
subterranean
WordNet sense 2 (beneath the surface):
subsurface
WordNet sense 3 (lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed)):
ulterior
WordNet sense 4 (secret or hidden; not openly practiced or engaged in or shown or avowed):
covert
From the ODE community, based on WordNet
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