In the latter part of the Middle Ages, much of Western Europe experienced a period of urbanization. Paris in particular had grown into one of Europe’s largest cities. As the population grew, so too did the city’s infrastructure. On the south bank of the River Seine, limestone quarries were mined, and provided ongoing construction projects – including the Notre Dame Cathedral and the Louvre – with stone. Once their resources were exhausted, the mines became neglected and fell into disrepair as the City of Light’s booming infrastructure continued to grow atop networks of delicate, forgotten quarries.

In 1774 the inevitable struck. Ironically, Rue d’Enfer (Hell Street) collapsed, swallowing a house along with it. Charles Axel Guillaumot, the architect of King Louis XVI, was tasked with exploring the underground tunnels and assessing the damage. Guillaumot discovered that many of the tunnels were unstable and could tragically cause several areas of Paris to collapse. Led by Guillaumot, the Inspection Générale des Carrières (Inspection of Mines) was created, and tunnel walls and ceilings were stabilized. But how the refurbished mines would next be used was still an unknown.

The fragile mines were not the only issue Paris was struggling to fix. The increase in population led to the city’s cemeteries becoming hugely overcrowded. The problem was so severe, that at times, the lack of space caused the uncovering of graves. Les Innocents (Holy Innocents) cemetery, one of the city’s oldest and largest, was seen as the heart of the problem. Neighboring businesses complained of strong foul odors deterring business. Orders from the King banning burials within the city were issued, but often ignored by local churches. The turning point came on May 30, 1780, after the city endured a long period of rain. One of the cemetery’s cellar walls collapsed, causing the corpses to spill out into the adjacent property. The cemetery had now become a public health risk. Unable to identify a solution, the city turned to the mines. It was now clear what they could be used for.

In 1786, the city began exhuming corpses from the local cemeteries, starting with Les Innocents. Over the course of 12 years, remains were unearthed from all Parisian graveyards and moved to the mining tunnels, creating an ossuary. Twenty meters below ground level are the remains of approximately 6 million bodies, some dating back more than 1,200 years. Along the tunnel walls, skulls and bones are stacked in a macabre display of art representing life and death.

Throughout the French Revolution several bodies were buried within the catacombs, including members of the Swiss Guard that were killed during the storming of Tuileries Palace on August 10, 1792. During World War II both the French Resistance and German soldiers used the tunnels as hideouts and bunkers.

Today, travelers can visit avenue du Colonel Henri Rol-Tanguy, in the 14th arrondissement, where almost a mile of the ossuary can be explored. Traveling back in time to uncover one of the city’s more eerie hidden gems brings visitors face-to-face with mortality – the welcome sign forewarning them, “Stop! This is the empire of death!”