I've been on the Seattle underground tour and I highly recommend it! The city history is fun, too, with all sorts of sordid stories.
Atlanta's shopping mall is cool, but I thought it was kind of creepy.
Chicago has some underground areas. I saw a program about it on (I think) the History channel.
In LA, the old trolley station and miles of tunnels were rediscovered quite by accident several decades after they'd been abandoned.
There are many, many cities in the U.S. and abroad that have been built upon the ruins of previous cities. Istanbul (formerly Constantinople) is probably one of the most extensive, containing extensive roman ruins within its sewers. The same is true of Alexandria, although I think most of the architecture beneath it is Greek.
You should be able to find plenty of information and examples on the internet. I think it's fascinating myself.
Gary