Long before the tunnels became associated with skulls and bones, they were quarries.

For centuries, limestone was extracted from beneath Paris and used as building material for the growing city above. In other words, parts of Paris were quite literally built using stone taken from underneath Paris.

As quarrying expanded, an enormous maze of underground galleries developed beneath the city. The abandoned quarry network eventually extended beneath an area of roughly 800 hectares.

There was, however, a fairly significant problem with hollowing out the ground underneath a growing city.

Eventually, parts of it started collapsing.

One particularly serious collapse occurred along Rue Denfert-Rochereau in 1774, when roughly 300 meters of roadway were swallowed up. In response to the growing danger posed by abandoned quarries, King Louis XVI created the Department of General Quarry Inspection in 1777 to map, stabilize, and reinforce the underground spaces beneath Paris.

And just a few years later, Paris discovered another use for those tunnels.

By the late 1700s, some Paris cemeteries had been accepting burials for centuries and had become dangerously overcrowded.

The largest problem was the Saints-Innocents Cemetery. It had been used for nearly ten centuries and eventually became so overcrowded that concerns about sanitation and public health could no longer be ignored. The cemetery was officially closed in 1780.

But closing the cemetery only solved part of the problem.

There were still centuries’ worth of human remains that needed somewhere to go.

Paris officials looked beneath the city and realized those abandoned limestone quarries offered a solution.

And so began one of the strangest relocations in Paris history: moving the dead through the city under the cover of darkness.

Beginning in 1785, workers started removing remains from Saints-Innocents Cemetery and transporting them to the former Tombe-Issoire quarries.

The transfers were carried out at night in part to avoid hostile reactions from Parisians and the Church. Bones were transported through the city, lowered down quarry wells, and then distributed throughout the underground galleries by quarry workers.

Just imagine being outside in Paris in the late 1700s and seeing carts carrying human remains quietly making their way through the streets after dark.

The transfers did not end with Saints-Innocents. Remains from other Parisian cemeteries were gradually moved underground as well. Major transfers continued through 1814, with additional deposits taking place again during later periods of redevelopment in Paris. The last major bone deposits connected with nineteenth-century urban development occurred in 1860.

They were not originally called the catacombs. The underground site was officially consecrated as the Paris Municipal Ossuary on April 7, 1786.

The name “Catacombs” came later and was inspired by the ancient Roman catacombs that had already captured the public imagination.

So technically, what visitors call the Paris Catacombs today began as an ossuary—a place created specifically to house human skeletal remains—inside a much larger network of former quarries.

That distinction makes the underground world of Paris even more fascinating.

The bones are only one chapter of a much bigger story beneath the city.

One thing that surprised me was learning that the bones were not originally arranged into the elaborate walls we see today, they were created later.

At first, remains were lowered into the quarries and piled within the galleries.

Beginning in the early 1800s, Inspector Louis-Étienne Héricart de Thury transformed the ossuary into something much more deliberate. From 1810 to 1814, the site was reorganized using carefully arranged skulls and long bones, monuments, plaques, inscriptions, and other decorative elements.

That explains why some sections almost look designed.

Because they were.

The skulls and bones forming the visible walls were intentionally arranged, creating patterns that are strangely beautiful until your brain reminds you:

Those are people.

Behind those carefully constructed walls are even more human remains.

Before entering the ossuary, visitors encounter one of the most famous inscriptions in the Catacombs:

“Arrête! C’est ici l’empire de la mort.”

Essentially: “Stop! This is the empire of death.”

Not exactly the warm “Bienvenue à Paris” you receive at most attractions.

But it immediately changes the mood.

You are not entering a haunted house or a recreated historical exhibit.

You are entering a burial place.

Throughout the Catacombs, other inscriptions, poems, and philosophical or religious texts were deliberately added to encourage visitors to think about life, death, and mortality.

It gives the experience a surprisingly reflective quality.

People have been touring the catacombs for more than 200 years.

The fascination with the Catacombs is hardly new.

The ossuary officially opened to visitors by appointment in 1809, and it quickly became popular with both French and international visitors. A guest register was even placed near the end of the route so visitors could record their reactions.

Some fairly notable people made the journey underground over the years.

The future King Charles X visited in 1787, Austrian Emperor Francis I toured the Catacombs in 1814, and Napoleon III visited with his son in 1860.

Apparently, people have been voluntarily descending underground to look at thousands of skulls for a very long time.

So at least I know I am in good historical company.

The part you visit is only a piece of underground Paris.

Today, the official Catacombs visitor circuit is about 1.5 kilometers long and covers approximately 11,000 square meters. It sits around 20 meters underground—roughly the height of a five-story building below street level.

Getting there requires 131 steps down.

Getting back to normal Paris requires another 112 steps up.

That is 243 steps total.

The average temperature underground is around 14°C, or about 57°F, and the tunnels can be humid, narrow, slippery, and dark.

What visitors experience, however, represents only part of the far larger quarry system beneath Paris. The ossuary was created within galleries belonging to that enormous preexisting underground network.

There is something fascinating about knowing that while millions of people go about their daily lives above ground, another Paris exists underneath them.

And then you remember these were real people.

The official Catacombs describe the ossuary as containing the remains of several million Parisians.

That number is almost too large to process.

When you first walk past the walls, your eyes naturally notice the patterns—the rows of bones, the skulls placed between them, the incredible scale of everything around you.

But then you stop looking at the wall.

You look at one skull.

And suddenly the entire experience changes.

That skull belonged to one person.

Someone who once walked through Paris.

Someone who had a family.

Someone who loved people, argued with people, worried about things, laughed, worked, ate meals, walked streets, and lived an entire life that we will never know anything about.

Then you look beside that skull.

And there is another.

And another.

And thousands more.

For me, that is where the Catacombs stopped simply being fascinating and became something much more powerful.

Above us was the Paris everyone recognizes.

The Eiffel Tower.

The Seine.

Beautiful buildings.

Sidewalk cafés.

Fresh baguettes.

People drinking wine and eating pastries.

And beneath all of it was this enormous reminder of the generations of people who had lived there before us.

That contrast may have been what fascinated me most.

Paris is called the City of Light, but underneath that beautiful city lies a much darker story—one involving geology, engineering, overcrowded cemeteries, public-health problems, nighttime bone transfers, and millions of people whose remains eventually became part of one of the most extraordinary places we have ever visited.

The Eiffel Tower may be the Paris everyone photographs.

But the Catacombs showed me the Paris beneath the postcard.