In AoW3 the underground was just one level, and to enter it, you used what were teleporters, dressed up to look like cave entrances.
Basically, it is possible in AoW3, in a custom map, to create as many layers as you want.
Now I don't know if this works the same way in Planetfall but I *think* it does, because there are teleporters in Planetfall, dressed up as teleporters.
When you put down a teleporter in AoW3 (as in made a map) you designated the exit coordinates.
Therefore, I *think* it works the same in PF.
If I am right, and bear in mind this is an educated guess, nothing more, then you should be able to create as many layers as you like, and name them whatever you like.
Developing this line of thinking, and referring to the community made shadow realm mod for AoW3, you should, in theory, be able to allocate a unit attribute linked to that plane, e.g. "weightlessness" in "space."
This is all conjecture.