The Catacombs are a creepy network of underground tunnels (former mines where stone used to build the city were excavated) that are now extensively lined with thousands of human bones. The bones were moved to the tunnels in the late 1700s, when the stench of rotting corpses in local cemeteries became more than Parisians could bear. Bones and bodies from cemeteries were moved to the catacombs at night during a "relocation" project that took several years. The result is a morbid subterranean necropolis of sorts. The low, narrow tunnels which make up the catacombs are lined with neatly (and often artistically) stacked human bones of every description. The effect is slightly disconcerting, but fascinating nonetheless.
Rumor has it that Charles X threw wild parties in the catacombs just before the Revolution, and the French Resistance brought the place to life during WWII when they set up their headquarters in the underground graveyard.
The passageways are given "street' names, and there are proverbs engraved into the walls at some points. I don't remember at this time what they said, but I do remember that they were often witty and ironic. At the entrance to the catacombs is a sign which reads "Stop! Beyond here is the empire of death!"
The catacombs are a lot of fun to visit. The history and creepiness of it all is fascinating - don't miss it! Admission is about 29F (there is also a cheaper student rate with proof of ID).