Catacombs of Paris

By the late 18th century, the cemetery of Les Innocents in central Paris had become a public health crisis: centuries of burials had raised the ground level by metres above the surrounding streets, and a wall collapse in 1780 spilled decomposing remains into a neighbouring property. Beginning in 1786, the city undertook a massive operation to exhume its cemeteries and transfer the remains — by night, in religious procession — into the disused limestone quarries that honeycombed the city's southern districts, a process that continued for decades and ultimately relocated the remains of an estimated six million people. In the early 19th century, the quarry inspector Louis-Étienne Héricart de Thury arranged the bones into the decorative walls of skulls and femurs that visitors see today, along with commemorative plaques and inscriptions.

The official ossuary, known as the Catacombes de Paris, comprises only about 2 kilometres of the more than 320 kilometres of tunnels that form the broader network beneath the city — the remainder of which is illegal to enter but is regularly explored by "cataphiles," an underground subculture with its own maps, parties, and even a dedicated police unit (the Cataflics) tasked with policing it. Over the centuries the tunnels have served as a Resistance hideout during the Nazi occupation, an air-raid shelter, and the site of an unsolved 1955 case in which a cataphile's camera was found alone in a chamber, developed years later to reveal photographs of tunnels never identified.