The l'Ossuaire Municipal is a series of tunnels stacked high with the bones of six million Parisians

Underneath place Denfert-Rochereau in Montparnasse stretch several kilometers of tunnels that are stacked with the bones and skulls of some six million dead Parisians.

Some of the bones used as mosaics in decorative motifs, other simply form grisly walls flanking the corridors.

OK: Undeniably creepy—and just a bit cool—but why?

Bring out your dead! The Paris bone collectors

Well, by the 18th century, Paris just had too many dead people. The cemeteries were full. The solution: a civic ossuary, or bone depository.

Paris was once riddled with underground gypsum quarries. (What did they make with the gypsum? Why "plaster of Paris " of course.) Many of these former quarries still honeycomb outlying parts of the city with networks of artificial tunnels.

Starting in the 1780s, some of these disused quarries in the Montparnasse district became a dumping ground for excess skeletons from city cemeteries and hospitals. It remained the prime city ossuary until 1814.

Welcome to hell in Paris

It is unclear whether these particular quarries in Montparnasse were chosen with a sense of humor, as they lie directly beneath the southern gate to the city known as the Barrière d'Enfer, or "Gate to Hell." regardless, they remain the only section of the huge network of tunnels under Paris open to the public.

You do have to descend a 19m (62-foot) spiral staricase to get into the catacombs, and the section of the tunnels open to the public meanders for about 2km (1.25 miles).

It's spooky. It's grisly. It's great fun for kids, too (once the're old enough not to get too creeped out by it).