Stop! This is the empire of the dead, reads a sign greeting visitors at the entrance to the two-kilometre long tunnels filled with the bones of approximately six million Parisians.
A place of fear, legend, inspiration and exploration, Paris' skull and bone-lined Catacombs draw thousands of visitors each year, many of whom queue for hours to explore the ghoulish burial site.
The transfer of human remains from Parisian cemeteries to the tunnels began towards the end of the 18th century when authorities realised the decomposition of bodies in the city's cemeteries was not particularly good for public health.
"It was said 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 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".
That's not surprising considering 800m of the journey is a walk between walls lined entirely with bones.
Among the stacked bones are 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 not survive until the morning," stated one.
"All of these tibias piled in such a decorative manner pose the question [of mortality]," said museum director Valerie Guillaume. "There's visual proof that everyone is equal in death that is quite philosophical."
Tourist Antonina Bodak, visiting from Belarus, believed it was a place like no other. "It's mysterious, exciting, strange. It has no equal," said Bodak. "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 200km and extends beneath a large part of the city.
"The term 'Catacombs', borrowed from the Roman Catacombs, can be confusing," said Robin.
It refers to all 800ha 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 held secret parties deep below the pulsating city or organised 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. But the dark, dank passageways came into their own during a heatwave in 2003, when tourists and locals scrambled down there to enjoy the relative cool 15C and authorities were forced to turn desperate people away.
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 such as Victor Hugo, Gaston Leroux and Anne Rice all drew inspiration from the famous underground tunnels.
And for the visitor, if the thought of walking through kilometres of bone-filled halls isn't frightening enough already, the museum has now extended its hours from 5pm to 8pm, allowing people to visit the tunnels after sunset.
The magic of Paris
If traipsing past walls of bones at the Catacombs isn't your scene, check out some of these top spots in the City of Light.
Get a bird's-eye view
It seems ironic that one of the ugliest buildings in Paris affords what is possibly the most spectacular view of the city. The Montparnasse Tower is a nondescript, 210m skyscraper but from the top deck you can spy all of the key monuments, parks and neighbourhoods of the French capital - on a clear day you can see as far as 40km across the city. There is also a small wine bar on site so you can quench your thirst.
Check out some art
If the queue to the Louvre makes your head spin, visit the vibrant Pompidou Centre instead. Boasting the largest collection of modern art in Europe, including the studio of revolutionary sculptor Constantin Brancusi, the centre features live performances, workshops and film screenings.
Go goth at Notre Dame
The gothic cathedral in the fourth arrondissement features magnificent stained-glass windows, imposing sculptures and distinctive flying buttresses. Construction started in 1163 and the building has suffered through various riots, wars and plunderings to become best-loved attractions.
Stroll the gardens
The second largest park in Paris,
the Jardin du Luxembourg, is the ideal place to sit back, relax and enjoy the scenery. Alongside the stunning floral gardens and tree-lined paths, the sweeping grounds contain hundreds of statues, monuments and fountains, and it's only a short walk from many of Paris' main attractions.
Graze at the food markets
Marche des Enfants Rouges is a bustling indoor food market, housed in what was once an orphanage. It has a huge range of food stalls, as well as more florists, fishmongers and organic artisan vendors than you can shake a stick at, so is a great place to meander and enjoy the atmosphere.
Gaze on luxury
The vast line-up of luxury stores might be beyond the wallets of most of us mere mortals, but if you're a fashionista, Le Bon Marche is well worth a look. Built in 1852, the city's oldest department store is the epitome of Parisian chic, and boasts a glorious food hall, La Grande Épicerie de Paris.