Paris might be the city of lights and love, but beneath its chic streets lies a dark labyrinth filled with the bones of 6 million Parisians.

Just like many other booming cities of the early industrial age, Paris was suffering from problems, specifically death and disease. The promise of life in the big city brought flocks of people rushing in from the countryside. Towards the end of the 18th century, after nature took its course, this situation eventually resulted in overstuffed graveyards and cemeteries.

Les Innocents, one of the biggest Parisian graveyards in the 1700s, reportedly oozed the stench of rotting bodies, their grounds unable to cope with the demand brought on by the city’s overcrowding. The smell was so bad, local perfumers were said to struggle to sell their goods. By May 1780, the cemetery was literally bursting at the seams. A cellar wall of a property bordering Les Innocents split open under the pressure of excess burials and spring rains, causing a gush of half-decomposed bodies and disease to flood into the basement.

Within months, authorities ordered the closure of Les Innocents and the city’s other cemeteries. No more bodies could be buried within the city. With the threat of public health still looming, the city also decided to remove the contents of the city’s current cemeteries.

Fortunately, there was a plan. Paris was once home to a number of old mines and quarries, perfect for an underground ossuary to store the city’s dead. Primarily between 1787 and 1814, bones were transferred deep into the mines. The entrance was built just outside the old city gate, the suitably named Barrière d'Enfer, which loosely translates to the "Gate of Hell”. While the skeletons were initially piled in quarries in a haphazard fashion, they were eventually organized into the neat placement you see today.

Among the 6 million skeletal remains in the ossuary, you can find dozens of characters from French history, including numerous beheaded figures from the French Revolution, such as Georges Danton and Maximilien de Robespierre, as well as celebrated artists, such as Charles Perrault, known for writing fairy tales like Little Red Riding Hood, Cinderella, and Sleeping Beauty.

Today, The Paris Catacombs snake 20 meters (65 feet) beneath the streets and you can still visit around 1.5 kilometers (0.9 miles) of the ossuary as a tourist. Venturing into the off-limits galleries has been illegal since 1955, but thrill-seekers have been known to delve deeper into the labyrinth through hidden entrances. However, that certainly can’t be recommended. In 2017, three teenagers were rescued from the depths of the catacombs after being lost within its maze for three days.