My understanding is that in Atlanta, several square miles of streets in and around downtown were raised from around 1900 to 1925. Underground Atlanta consists of only a few blocks of restored original streets, now underneath the viaducts used to raise the street level.
The one time I was in downtown Atlanta, I saw the ends of several viaducts, with a void underneath, on the edge of the downtown street grid. I’ve never heard of any vadding underneath the viaducts outside the Underground Atlanta attraction, though. Are there any Web sites that document the hidden Underground Atlanta? Anybody ever been down there?