This is just the thing for anyone with morbid interests: a descent through dark, clammy passages brings you to Paris's principal ossuary, which also once served as a hideout maze for the French Resistance. Bones from the defunct Cimetière des Innocents were the first to arrive in 1786, when decomposing bodies started seeping into the cellars of the market at Les Halles, drawing swarms of ravenous rats. The legions of bones dumped here are stacked not by owner but by type—rows of skulls, packs of tibias, and piles of spinal disks, often rather artfully arranged. Among the nameless 6 million or so are the bones of Madame de Pompadour (1721–64), laid to rest with the riffraff after a lifetime spent as the mistress of Louis XV. Unfortunately, one of the most interesting aspects of the catacombs is one you probably won't see: cataphiles, mostly art students, have found alternate entrances into its 300 km (186 miles) of tunnels and here they make art, party, and purportedly raise hell. Arrive early as the line can get long and only 200 people can descend at a time. Audioguides are available for €5. Not recommended for claustrophobes or young children.