I remember an article in Dragon that talked about designing a realistic castle. Step one was locating a USGS contour map. As this was before the internet, I drove myself crazy trying to track one down. Things are so much easier now.
I'm using old maps of Grand Canyon to make a "mega dungeon" More like Caves of Chaos made really big, like grand canyon big.@sirlarkins I spent many hours when I was a freshman at KU's Library's Map room. So many awesome maps. Librarian was confused, but happy, some one was interested in all these Norwegian topo maps they had (I was working on a viking campaign (or at least something with fjords!))