The Paris Catacombs draw thousands of visitors a year, but few know the macabre tunnels' unusual history. While the tunnels are named after the Catacombs of Rome, which were built in the first century by Christians and Jews forced to perform their burial rites in secret, the catacombs of Paris were founded in the 18th century in response to two secular problems: sinkholes and a surplus of dead bodies.

Notre Dame. Les Invalides. The Louvre. Many of the City of Light’s most iconic buildings are made of Lutetian Limestone, or “Paris Stone,” which has been mined from quarries on the outskirts of Paris since Roman times. By the late 18th century, the city had expanded so greatly that much of it was built directly above the old mines, leading to unstable conditions.

In 1774, a massive sinkhole in the Rue de l’Enfer (“Road to Hell”) engulfed houses, carts, and people, who fell over 84 feet to their death. Multiple sinkholes over the next few years caused panic and outrage. In response, King Louis XVI created the Inspection Générale des Carrières, or IGC, in 1777 to map and maintain the quarries. Chief Inspector Charles-Axel Guillaumot began a race against time to locate material to shore up the vacant mines. A growing public health problem provided the grim solution.

In the 18th century, most Parisians were buried in communal graves on the grounds of their parish church. The graves were left open until they were full, a process that could take months.  “The bodies would break down over the course of five years, then the grave would be re-opened, the bones extracted and moved to a charnel house,” says Dr. Erin-Marie Legacey, author of Making Space for the Dead: Catacombs, Cemeteries, and the Reimagining of Paris, 1780–1830.

The largest burying ground in Paris was Holy Innocents' Cemetery, which had been in continuous use for over 500 years. As the city above grew, overcrowding became an issue down below and by the mid-1700s, 1/10 of the city’s dead were buried there each year. The living and the dead jostled for space, especially where the cemetery bordered the bustling Les Halles Market. Historian Rosemary Wakeman writes: “Human decomposition mixed with the blood and guts of the market, with piles of rubbish to form a putrid stench, a dangerous effluence that made Les Halles an axis of infection and disease.”

French writer Louis-Sébastien Mercier warned that “cadaverous miasmas threatened to poison the atmosphere” of the city. He claimed the vapors soured milk and wine that was stored nearby and that the “cadaverous humidity” of the cemetery’s walls had “the effects of venom.”

“Miasmas can be roughly translated as contaminated air that was thought to spread disease,” says David Barnes, author of The Great Stink of Paris and the Nineteenth-Century Struggle against Filth and Germs. While germ theory didn’t yet exist, “It was common knowledge that people who lived and worked in areas near cemeteries or slaughterhouses or any major centers of decaying organic matter were more likely to be sick,” says Barnes.

The growing concern with sanitation coincided with the declining power of the Catholic Church in France. In 1765, the Parlement of Paris condemned church burial for almost all citizens and proclaimed that burials taking place after January first needed be in new cemeteries outside of the city limits.

“Republicans in France saw themselves as the vanguard of the future of civilization and the Church as the antithesis of everything that was modern, civilized, and healthy,” says Barnes. “Cleaning up the cities, villages, and countryside of France was a political project framed as a scientific and secular health crusade.” The law was unpopular with the faithful who wanted to be buried where their families had been laid to rest for centuries.

It took a new crisis to seal the fate of Parisian cemeteries. In the spring of 1780, residents of the rue de la Lingerie on the Western edge of Holy Innocents’ began to complain of respiratory issues, vomiting, and delirium. When inspectors came to investigate, they found that gases from decomposing bodies had burst through cellar walls and risen to the ground floor of at least three homes. A Royal Ordinance declared Innocents a threat to the city and closed it down.

In December of 1785, workers began exhuming bodies from Holy Innocents’ at night and carting them by torchlight to their new resting place: The city’s catacombs. It was a marriage of convenience, with the vast underground mines offering a local (and more sanitary) storage solution. The “Paris Municipal Ossuary,” as the catacombs were officially named, was consecrated on April 7, 1786.

The outbreak of the French Revolution in 1789 put a temporary halt to Guillaumot’s plans, but as bodies piled up during the Reign of Terror, the city’s dead began being directly buried in the catacombs, which had gained a new reputation as “The Empire of the Dead.”

There are no headstones or markers for individuals in the catacombs; Parisians from different classes and centuries are laid to rest side-by-side. “The catacombs are a space of equality. You have the bones of nobles next to a pauper and you can’t tell the difference. Equality in death had long been a Christian message, but in the wake of the French Revolution, it also has resonant political undertones,” says Legacey.

The bones from over six million bodies were shaped into archways, tunnels, and walls. One 19th century visitor recalled:

“Bones bend into arcs [and] rise into columns, an artistic hand created a kind of mosaic out of these final remains of humanity, whose ordered regularity only adds to the profound contemplation that this space inspires…Here, all distinctions of sex, wealth, and rank have finally disappeared.”

The Paris Catacombs have been a tourist attraction since Napoleon Bonaparte ordered them opened to the public in 1809. For some, it was a space to mourn the violence of the Revolution, “especially in front of bones of notorious events like the September Massacres, when over 1,000 people were killed in four days,” says Legacey.

Others sought thrills and entertainment on their candlelit tour. “It was a way to safely confront death and violence in the aftermath of a tumultuous, high anxiety period,” says Legacey.

The French obsession with the frights of Gothic fiction and the macabre could be viscerally felt in the cool, underground caves lined with anonymous bones. Visitors could observe the black line on the ceiling of the Paris Catacombs meant to guide anyone who got separated from their tour—lines that would be invisible should your candle go out.

Only 1.5 km of the 300 km of tunnels that form the Paris Catacombs are filled with bones. The rest of the tunnels have been used for everything from subterranean mushroom farming to beer storage to serving as a bomb raid shelter and meeting place for the French Resistance during World War II.

Since 1955, it has been illegal to enter the Catacombs without permission, though explorers known as “cataphiles” descend into the darkness to explore the hidden world beneath Paris. A special branch of the Paris police, dubbed “cataflics” by locals, patrol beneath the city streets. In 2004, officers uncovered an illegal underground cinema and bar.

As a liminal space beneath the ever-changing city, a big part of the Paris Catacomb’s appeal is their remove from the bustle of the city above: “Since they’ve been opened to the public, people thought about the catacombs as a space out of time,” says Legacey.

The 18th century opened with an unprecedented amount of upheaval: “Everything changed, from names of streets to the way people measured time. Accelerating technology during the Industrial Revolution quickened the pace of change. As soon as you go underground, that’s all missing,” she says. “In the 21st century, cell phones don’t get reception down there. To descend into the Catacombs is to travel to the center of Paris and back in time.”