The Catacombs of Paris

Under Paris are the emptied limestone quarries that built the city, and in 1786 the authorities began moving the contents of overflowing cemeteries down into them, six million people, stacked as walls of femurs and skulls arranged in patterns by quarry workers with an eye for the macabre. A small lit section is open as a museum; the wider three-hundred-kilometre network is illegal to enter, and cataphiles who do it anyway occasionally get lost or drowned. PUBLIC museum section only, Paris.