Subtropolis
I've been on the subway tour with the Cincinnati Museum. You can still see the entrance tunnels off of I-75. The ones on the I-71 side are pretty much lost to development.
Construction stopped with the start of WWI. Funds for the project were reallocated and the subway abandoned. A massive water main runs thru the center of one of the train tunnels and the stations were used a fallout shelters during the cold war.
The homeless pretty much have run of the place now.