Subterranean : existing, situated, or operating below the surface of the earth; underground.
Informally, we can use subterranean to describe that which lies below the surface; technically, terranean refers to terra, the Latin name for Earth.
What word could we use to mean that which lies below a surface, in general. I.e., what would we say about caves under the surface of Mars, “submartian”?
This question is predicated on the wrong definition of “Terra”. As seen at Wiktionary:
- ground, dry land
- earth, soil, dirt
That is, not “Earth” (the planet) but instead “earth”, meaning “dirt” or “ground”.
So, subterranean is acceptable to describe anything underground, whether under the surface of Planet Earth or not.
Edit: I would not suggest using “subsurface” unless it was acceptable to include underwater things such as a submarine.