Underground Provinces.

[Edi]

Originally Posted by Gandalf Parker

The subterrainian part is already in the game.
But the finding entrances isnt. Im not sure if that can be done

First off, this is a question of mapmaking. Not modding. The two things are similar, but they work with some very significant differences.

Cave provinces exist in the game, but hidden entrances don't. There is no way to implement this in a map file either, since the neighbor relationships between cave provinces and any other provinces work exactly like normal neighbor relationships between e.g. plains and forest.

One of the best examples of cave province implementation in a map that I have seen can be found in Zepath's World of Warhammer map. Twan's Greyhawk map (Oeridia) also has an extensive underground section, which is implemented as a separate section on the side of the map instead of being mixed in with the other provinces (which is how World of Warhamer does things).

So those two maps will give you two examples of how to do things as much as it is possible. You can find them linked in the map list.