"Do you enjoy dead people, sir?" enquired the concierge. It was an unusual question.

"Because if you do," she continued, "I can recommend the Denfert-Rochereau Ossuary. It's extremely enjoyable to visit — seven million corpses are interred there."

The concierge took a map from her desk, and drew a smiley face to mark the location. "You haven't really been to Paris until you've entered the kingdom of the dead," she said.

As luck would have it, Denfert-Rochereau was only a few kilometres from my apartment. It was just a short stroll down the Avenue du Général LeClerc — followed by a long hour of searching for the exact location of the ossuary. The smiley face drawn by the concierge covered about five blocks, and the city authorities had obviously decided that signposting seven million corpses would be in poor taste. My occasional attempts to ask for directions were hindered by my inability to say "mass grave" in French.

I eventually discovered the entrance, which was hidden in a tiny park in the middle of Ledoux Square. Appropriately enough, a pair of black doors crowned the staircase which led beneath the city. In the centre of the right-hand door was a small sign which read: "Entry to the Catacombs".

The Denfert-Rochereau ossuary was established in 1786 as a response to the overcrowding of city cemeteries. The circumstances leading to its creation were rather unpleasant. By the end of the 18th century, the number of corpses produced by the thriving Parisian population had raised the ground level in some churchyards by over six metres. Fluids from the decomposing bodies seeped into the streets and waterways, causing major sanitation problems. The quantity of cadavers was so immense that in 1780, a wall at the Cimetière des Innocents actually collapsed, and crushed a number of local inhabitants beneath the decomposing flesh of their relatives.

The city authorities decided that a municipal ossuary would be the answer to their problems. Bodies would be kept buried in the cemeteries only until the soft tissues had decayed. After this, the bones would be disinterred and transported to their final resting place. The word 'ossuary' (from the Latin word ossis for bone) is the name given to a repository for human skeletal remains.

Entering the black doors, I was confronted by an elderly ossuary employee sitting in a ticket booth. There were no other customers in sight. A dusty notice on the booth warned that: "Children may be disturbed by the contents of the ossuary!". Underneath was a smaller sign which read: "Free entry for infants". I purchased a ticket, and was directed to a narrow stone staircase in the corner of the room.

The staircase was clammy and confined, and spiralled downwards into the earth like a gigantic corkscrew. The last steps — some 25 metres below street level — led into a cramped tunnel. The ceiling was low, and the walls were barely wider than my shoulders. Water oozed from the stonework.

I had reached the catacombs — a network of quarry tunnels dating back to Roman times. For over a thousand years the excavation of these caverns provided the construction materials for the roads and buildings of Paris. Eventually they formed a labyrinth over 300 kilometres in length. Although I was only a first-time visitor, I could easily see that this was an ideal location to dispose of seven million dead bodies.

A sign on the wall informed me that this tunnel had been a major transportation corridor for the ossuary. It took the city authorities nearly 14 years to empty the bloated cemeteries of Paris. The skeletons were delivered each night by special priests chanting the office for the dead: the bones of the poor were carried en masse in wagons; the bones of the rich in ornate catafalques.

As I made my way along the narrow tunnel, I wondered how much of the ossuary the public would be permitted to see. Some sample bones in a glass display case, or perhaps a few innocuous photographs? The catacombs were already dank and sinister enough for my tastes. I certainly didn't feel the need for a close encounter with any actual dead people.

The sickly cloying smell hit me as I rounded the first corner. It wasn't exactly revolting, but it was certainly overpowering. And it clearly didn't originate from any natural subterranean source. My imagination went into overdrive — what on earth could it be? The marrow rotting inside bones? A special fungus that grew on human remains? Perhaps something emitted by the plague victims who had been interred in the ossuary?

I began to feel decidedly ill. The smell increased in strength as I walked further down the passage. It was hauntingly familiar, and yet frustratingly elusive. It wasn't until I turned another corner that the origin was revealed — a couple of teenagers smoking a joint. They pressed themselves against the wall to let me pass, giving me a close-up view of their massively dilated pupils.

"Whoa," said one of them in English, taking another drag on the joint as I squeezed awkwardly past him. "Those skeletons are going to be so trippy."

After I had walked another hundred metres the tunnel widened into a low-ceilinged cavern. The temperature seemed to have fallen by several degrees, and I could see my breath. A large opening gaped in the far wall — disturbingly reminiscent of an enormous mouth. On either side of it stood a stone obelisk. A welcoming message was carved into the lintel above the opening: "Halt! This is the Kingdom of the Dead".

The first thing I noticed when I stepped through the doorway was the humidity. Although it was quite cold, the atmosphere was also astonishingly wet. Water dripped almost continuously from the roof, and the rock floors were covered in a slimy mud.

The next thing I noticed was the unusual walls, which seemed to be constructed from some kind of quilted stonework. Every so often the texture of the walls was interrupted by a group of lumps. Closer inspection revealed that the lumps were human skulls arranged into neat geometric patterns. It seemed that one of the ossuary workers was also a budding artist.

Feeling rather queasy, I began to wander through the ossuary. It was only after I had progressed some 20 metres down the passageway that I made a nasty discovery. The walls weren't constructed from quilted stonework at all. They were actually comprised of human femurs. I was surrounded by hundreds of thousands of skeletons.

What I had initially taken for a tunnel was actually an aisle running down the middle of a cavern filled with human bones. The bones were stacked about three metres deep at the walls. It was a chastening moment. This is how we all end up. In the next half-kilometre, I would walk past the final product of millions of human lives.

The skeletons were organized into groups from different cemeteries — although the remains from certain large-scale fatalities had been assigned their own separate areas. Within each sign-posted section the bones were randomly intermingled. Somewhere in this ossuary Madame de Pompadour (the famous mistress of King Louis XV) was, quite literally, rubbing shoulders with ordinary Parisian commoners.

Above: Ossuary wall (photo: David Haywood).

A large stone inscription in one of the caverns bore the words: "Silence Mortals!". In my subdued state of mind this seemed like an unnecessary request. A few minutes later, however, the sound of laughter echoed through the ossuary. A young couple had just entered, and were preparing to snap a photograph. The man looked through his camera as the woman made kissing noises, and pressed her cheek against a mouldering skull. I wondered whose skull it was, and how they'd feel about ending their days as a prop for comical photography.

The desire to make merry in the ossuary is evidently quite common. In the nineteenth century it was a fashionable venue for underground dinner-parties. On one such occasion, an orchestra dressed in tuxedos performed a concert for a hundred members of the Parisian elite. Personally I couldn't imagine a less suitable setting for a good time. The ossuary workers seem to have agreed with me — the inscriptions they had carved into the walls weren't exactly cheerful. I was particularly taken by this uplifting message:

Thus, inevitably, the realm of men shall pass,
Virtuous or wicked: all life must expire,
Mankind is a sickly flock,
And time, that dismal shepherd, leads us to the grave.

The last section of the ossuary contained the remains of those massacred during the Paris riots of the 1780s. This was followed by another narrow tunnel with an oppressively low ceiling. From time to time the dripping roof was pierced by a cathedral-like gallery — which provided a welcome respite from my feelings of claustrophobia. Until, of course, I realized that these were places where the roof had previously undergone a massive cave-in.

Finally, some two kilometres from where I had entered the catacombs, I arrived at a long staircase which led upwards. Another pair of black doors returned me to the streets of Paris. Out into the sunlight, and back to the realm of the living.