Synonyms for Subterranean (same or very similar meaning)
WordNet sense 1 (being or operating under the surface of the earth):
subterraneousWordNet sense 2 (beneath the surface):
subsurfaceWordNet sense 3 (lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed)):
ulteriorWordNet sense 4 (secret or hidden; not openly practiced or engaged in or shown or avowed):
covertFrom the ODE community, based on WordNetadd/edit