In 1899, the core of downtown Seattle burned to the ground. While the shops quickly rebuilt & re-opened, the city itself took the opportunity to rebuild the streets some 36 feet higher than they previously had been (ostensibly to combat water pressure/sea level issues), meaning that pedestrians climbed ladders to go between street level and building entrances. Eventually, the city laid down sidewalks up on the new street level, and the underground city was all but forgotten. Today, via a building in Pioneer Square, you can still tour what remains of the abandoned underground, looking up at the people above through the opaque glass sidewalk.
The catacombs of Paris are an immense maze of tunnels dug under the city. In 1786, all the bodies from Cimetiere des Innocents were exhumed and moved into the tunnels. A sign above the door reads: Stop! Here is the empire of the dead... For a significantly less creepy (and infinitely cool) city under the city experience, check out the Seattle Underground tour.