Idea: Underground provinces

[Nerfix]

I wonder if it would be possible to add an Underground terrain type for maps with some sort of underworld provinces? This terrain type would always have Darkness in it. Poptypes could include Pale Ones, Shades and perhaps even new stuff like Olms or fungi creatures if IW would bother. Or maybe there wouldn't be poptypes but people making underground maps should take care of it themself.

All IW would need to do is the add an Underground terraintype, make a battle background for it (and perhaps a submerged undergound background too) and make it have Darkness on when battles are fough there.

I would really, really like to see this, as I have an idea for mod that would take largely place undergound. It would have fancy stuff like Olms, bat people and ctulhoid fungi in it.