The Dead Of Catacombs of Paris, France

After a prolonged bout of heavy rains flooded and unearthed the overcrowded Les Innocents cemetery in the spring of 1780, a wave of rotting corpses tumbled onto the property next door.

According to Smithsonian Mag, this horrifying event started a 12-year project to move bodies from Paris's cemeteries down into the city's former limestone quarries, eventually packing the underground tunnels with some 6 million bodies.

Today, about a mile of the subterranean labyrinth is open to visitors, who can take tours of the tunnels and artfully arranged displays of bones.