The remains of more than six million people are buried in a vast network of tunnels below Paris, France. People who call themselves 'cataphiles' visit the catacombs illegally and occasionally hold underground parties.AFP/Getty Images
Skulls and bones are arranged to form the walls of the tunnels in the Paris Catacombs. Circa 1900.Roger Viollet/Getty Images
Bones from the Saints-Innocents cemetery were moved to the catacombs after the cemetery began overflowing with remains.Roger Viollet/Getty Images
Parts of the catacomb network date back to medieval times. AFP/Getty Images
These subterranean quarries that make up the catacombs had been used to store the remains of generations of Parisians in a bid to cope with the overcrowding of Paris' cemeteries at the end of the 18th century.AFP/Getty Images
A pile of bones and skulls sits in the Paris Catacombs. Circa 1935Roger Viollet/Getty Images