[BigJMoney]

Yeah, I think this is a good idea. I know there haven't been any official maps that have sported underground layers, but there could be in the future. The terrain would simply be called "subterranean". I would think it would have the lowest inherent wealth and production bonuses, but would have a very high chance of things like gem mines, gold mines and iron mines. The only complication I can think of is that there would likely be many magic sites that currently have terrain type "any" that would not make sense being underground. You'd probably have to add a whole new property to magic sites called "terrain restricted" just because of this.

=$= Big J Money =$=