In the late 1700s, overflowing mass graves prompted King Louis XVI to move 6 million dead bodies from Paris cemeteries to underground quarry tunnels, which had originally been mined for limestone to build the city.

Lined with bones dating back to the 5th century, the Paris Catacombs opened to the public in 1809, and while most of the subterranean labyrinth remains illegal to enter, it has drawn the fascination of tourists and locals alike ever since.