The Catacombs in Paris is a huge maze of tunnels dug under the city. In 1786, all the bodies from the main cemeteries (something like 7 million) were exhumed and moved into these tunnels. At first the bones were just thrown in, but later they were arranged neatly and artistically as interest spread about this fascinating yet morbid attraction.
It is easy to find--directly across the street from Metro station Denfert-Rochereau. But be aware of the limited opening times--when we went, it was only open from 2 to 4pm.
The tunnels are unexpectedly extensive, and we were walking through rows and rows of bones--you walk for approximately 1km! Also, there are a lot of stairs when you first descend into the tunnels, and then when you leave, you have to climb up them again.
BRING A SMALL FLASHLIGHT—the lighting is very dim and it is hard to make out details, but the flash on your camera will illuminate your photos.
From journal Paris in February