Paris Catacombs
In the mid-1700s, Paris grew rapidly and her streets surged with construction. Stone was quarried from beneath the streets, which conveniently left a vast vault to accommodate the overcrowded cemeteries. The dead were transferred from battlefields, hospitals, and graveyards so full the outer walls sometimes burst, pouring bodies into the streets. The vaults continued to serve as a repository for remains throughout the 19th century until the catacombs housed some 6 million dead. Today you can tour Les Catacombes de Paris.

While it may not suit everyone's taste, this is definitely one of the more eerie tours in Europe. The entrance is within steps of the Denfert-Rochereau Metro stop in Montparnasse - a steep, circular staircase befitting a dungeon. The stairs descend 20 meters to a tunnel of claustrophobic dimensions. Dimly lit passageways are lined with bones. Walls of bones. Femurs and fibulas packed like bricks and adorned with circles, crosses, hearts, and fleurs de lis-like patterns made from skulls. The bone-flanked tunnels wind endlessly, interrupted by the occasional death-related quote or religious shrine.