"Subterranean", but for planets/surfaces other than Earth

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"?

Solution 1:

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.

Solution 2:

NASA uses subterranean, as in this press release:

NASA’s Hubble Space Telescope has the best evidence yet for an underground saltwater ocean on Ganymede, Jupiter’s largest moon. The subterranean ocean is thought to have more water than all the water on Earth's surface.

Granted, the OP asked for an alternate to subterranean, and NASA is perhaps not a leading expert in English, but subterranean seems preferable to a plethora of words specific to every planet and moon on which underground water might be found. This is especially so now that exoplanets are being found in great number, and the technology to find out if water exists on or inside them is sure to be developed, if it has not already been.