I'm contemplating creating a map for the capital of the major Dwarf empire in my epic campaign.

I've never done an underground city for some reason. The few villages we encountered I just placed in some random dungeons I wasn't going to use for anything else. But I want this city to look real, and not be in a dungeon but instead a real city underground.

Has anyone here done this before? I think a lot of things would have to be different to suit different economic needs.