Being or operating under the surface of the earth:
— Subterranean passages.

synonym: subterraneous.

Roget 208: deep, deep seated; profound, sunk, buried; submerged etc. 310; subaqueous, submarine, subterranean, subterraneous, subterrene^†; underground.    bottomless, soundless, fathomless; unfathomed, unfathomable; abysmal; deep as a well; bathycolpian^†; benthal^†, benthopelagic^†; downreaching^†, yawning.    knee deep, ankle deep.    ... show more

Lying beyond what is openly revealed or avowed (especially being kept in the background or deliberately concealed):
— Subterranean motives for murder.

synonyms: subterraneous, ulterior.