Subterranean is an adjective that describes something just below what can be seen, like the subterranean jealousy you hide under a smile and kind words for the actor who got the part you wanted.
Subterranean feelings and intentions are those you keep "on the down low." In fact, real things that are subterranean really are down low — underground, in fact. A subterranean worm lives under the earth's surface. A subterranean lair is a secret hideout dug under the dirt — or maybe it's just your basement. The word comes from the Latin subterraneus, from sub meaning "under" and terra meaning "earth."