If you’ve ever visited Paris, you know it’s an intensely colorful city full of life. But beneath this bustling metropolis, there’s a vast city of the dead.

The Paris Catacombs are home to the remains of over six million souls. They weren’t always a resting place for the dead. They were originally limestone quarries, opened in the 1st century AD to provide stone to build the city. Most of the churches, bridges, and buildings, including the Louvre and Notre Dame, are sheathed in the white stone. The underground quarries were abandoned and forgotten by the 14th century.

In the late 18th century, the city of Paris faced a serious public health problem. The cemeteries within the city walls were overflowing, and the stench of death was becoming unbearable. In an effort to solve this problem, the authorities decided to move bones from the cemeteries to underground quarries. The transfer began in 1785, and it took several years to get all the remains in their new resting place.

We were among the half a million curious tourists annually who make their way down the steep 131 steps to the dimly lit passageways. Uneven slippery floors wend past stacks of countless skulls and bones that stretched more than a mile. Several times I worried I might slip and crash into a dozen or so former Parisians.

After a while, the morbid uniqueness wore thin. It felt creepy peeping at the remains. At the end of an hour in this subterranean haunted realm, you look forward to the calf-burning 112-step climb back to the Paris we know and love. It’s definitely a one-and-done experience.