[st.patrik]

not to pour cold water on your idea, but an 'underground' terrain type really doesn't make sense, considering that the map is a bird's eye view of the land. Looking down from above you wouldn't see 'underground', you'd see whatever is over top of the underground (i.e. fields, mountains, forests, etc.)

With that said, I *believe* that Agartha's cities are 'cave cities' wherein it is always dark etc. - the idea being that they are, in fact, in great caves (i.e. underground). I see no reason (in theory) why you couldn't have an independant cave city, complete with underground-dwelling creatures, but that would be in a province that had some kind of surface to it (forest, mountain, etc.)