Idea: Underground provinces

[Nerfix]

Caduceus said:
Maybe a second layer of map with entrances and exits?

Alternatively, the Air nations could have an "air map" above the world...

Indeed, but that would require rather profound changes to the game. Maybe in Dom 4.

It's easier just to add a new terrain type. I'm asking this not only because of my own interest but also because I remember that there has been some "underworld" maps.