"Stop! This is the empire of the dead", reads a sign greeting visitors at the entrance to the two-kilometre (1.2-mile) long tunnels filled with the bones of approximately six million Parisians — although that figure is still debated.
The transfer of human remains from Parisian cemeteries to the tunnels began towards the end of the 18th century when authorities realized that the decomposition of bodies in the city's cemeteries was not particularly good for public health.
"It was said that the wine was turning bad and the milk was curdling," said Sylvie Robin, the site's curator.
So the city decided to move the skeletal remains to abandoned underground quarries.
And never short of a money-spinning idea to attract tourists, the city of light opened up its darkest corners to visitors shortly afterwards.
At the entrance of the site at Denfert-Rochereau in the south of Paris is a panel warning in three languages that the visit "is likely to upset especially sensitive people and children", not surprising considering that 800 metres of the journey is a walk between walls lined entirely with bones.
Among the stacked bones are also galleries with pictures and quotes that would give anyone pause to reflect on their own mortality — "Think in the morning that you might not survive until the evening, and in the evening that you might survive until the morning."
"All of these tibias piled in such a decorative manner pose the question (of mortality) … there's a visual proof that everyone is equal in death that is quite philosophical," said Valerie Guillaume, museum director.
'Mysterious, exciting, strange'
Tourist Antonina Bodak said it was a place like no other.
"It's mysterious, exciting, strange. It has no equal," said Bodak, who was visiting from Belarus. "I've always wanted to see this place."
Only a small portion of the tunnels is open to visitors. The entire length is more than 200 kilometres, and extends beneath a large part of the city.
"The term 'Catacombs', borrowed from the Roman Catacombs, can be confusing," said Robin, because it refers to all 800 hectares (nearly 2,000 acres) of quarries and not, contrary to popular belief, one massive cemetery.
Robin said the tunnels were originally used by people who wanted to avoid tolls when entering the city, and by smugglers and criminals who wanted to evade authorities.
Today, modern-day daredevils, known as "cataphiles", take exploration into their own hands, and nose around in the off-limits sections of the network of tunnels.
Some have gone as far as to hold secret parties deep below the pulsating city or even organize clandestine movie screenings.
Exploring the unofficial Catacombs has been against the law since 1955, and a special police squadron was set up in the 1990s to track down the "cataphiles," resulting in a macabre game of cat-and-mouse among the bones.
Those nabbed roaming the Catacombs without permission are fined a relatively light 40 euros ($50).
But the dark, dank passageways came into their own during a heatwave in 2003, with tourists and locals alike scrambling down there to enjoy the relative cool.
Authorities were forced to turn desperate people away seeking to escape record high temperatures for the relief of the Catacombs with their mean daily temperature of 15 degrees C (59 degrees F).
The Catacombs still exert a powerful influence on popular culture and have given rise to many urban legends, including tales of Masonic meetings, black masses, Nazi gatherings, gang fights, and serial killers.
It's no coincidence that writers like Victor Hugo, Gaston Leroux and Anne Rice all drew inspiration from the famous underground tunnels.
And for the visitor, if walking through miles of bone filled halls wasn't frightening enough already, the museum recently extended its hours from 5:00 pm to 8:00 pm, allowing people to visit the tunnels after sunset.