Idea: Underground provinces

[silhouette]

This would be a good addition I think, and it need not interfere with the 2-D nature of the maps. For example, there are impassable mountains which cannot be crossed or inhabited, but perhaps there can be a province underneath which is a path under. Impassable mountains don't need to be just a border feature, but can be a province with an underground instead.

There is also no restriction on making the province boundaries visible or cover 100% of the visible map. I can imagine a lake province connecting two other bodies of water, and also an underground cavern connecting the two land masses underneath the water, so the only trick would be how to display it on the 2-D map view and make it clear with neighbors, etc.

Sill