An underworld empire that invites tourists to see how death could have undone so many

Ever since the Paris catacombs were opened at the beginning of the last century, tourists have flocked to the doorway marked "Arrete! C'est ici l'Empire de la Mort" (Halt! Here is the Empire of Death).

The French king Charles X held a picnic here. Later visitors included Napoleon III and Bismarck. In 1897, a small orchestra sneaked in one night to play Chopin and Beethoven funeral marches and SaintSaens's Danse Macabre in a high society concert.

With the French capital silent as a tomb after le grand depart and the weather suffocating, I finally determined to visit the nether world of Paris. At least it would be cold in the catacombs, 20 metres below ground.

"To prevent stealing, bags will be checked at the exit," a sign warns at the entry. "People try to steal bones, especially students," explained an Indian from French-speaking Pondicherry at the ticket counter. The catacombs, the world's largest necropolis, are the property of the City of Paris, but most of the staff are from Pondicherry. They told me that French people refuse to work here.

Down 130 winding steps, I walk through the faintly-lit corridors of the ancient quarries that provided stone for Notre Dame Cathedral. A black line was torched onto the ceiling by quarry workers to lead them back out. My thoughts go naturally to Philibert Aspairt, a doorman who entered the labyrinthine quarries in 1793 - probably to look for wine stored in a convent cellar - and whose skeleton was found 11 years later, the keys still hanging from his leather belt. Amid the French Revolution, no one worried about his disappearance.

It is here, before the distraction of the ossuary and its engraved plaques, that claustrophobia threatens. "Twice I've had to lead women out of here," says another gentleman from Pondicherry. He wears galoshes for his rubbish-hunting treks through the 1.7 km maze. "People panic; they think they can't breathe. There was an Englishwoman who put her hands over her eyes and just screamed."

It is almost a relief to arrive at the famous lintel inscribed with the poet and abbot Jacques De lille's warning about the Empire of Death. Simple black and white designs, obelisks and diamonds - half pharaonic, half Alice in Wonderland - adorn the walls.

Against the wishes of the Church, six million skeletons were moved here between 1786 and 1814, most from the cimitiere des innocents site of the present-day Forum des Halles shopping centre and for six centuries Paris's main burial ground. The stench had become unbearable for nearby residents. So the bones were dumped into the old quarries through a well-shaft equipped with a chain that was pulled back and forth to prevent it from clogging.

An early 19th century engineer with a passion for order had the mountains of bones arranged in piles as neat as winter kindling.

Rows of skulls break up the monotony, some staring ghoulishly at you as you walk around a corner, some polished to a shine by thousands of luck-seeking fingers. Some look fragile as eggshells, with holes in them. "I never would have believed that death could have undone so many," wrote Dante in the Divine Comedy.

The scale of it is awesome - 30-ft deep walls of bones, brown with age. Bones behind wrought iron gates, stacked down corridors as far as you can see; a whole city of the dead. The alleyways bear signs corresponding to the streets 20 metres above.

These macabre decorations are loosely held together by grey mortar, though I see one wall that has collapsed down a side passage. The pile of cast-off tibias looks like the kitchen after my cat attacks our roast chicken.

Hericart de Thury, the order-obsessed engineer, was also responsible for placing throughout the catacombs hundreds of engraved plaques tracing the churchyards of origin and quoting ancient Greeks, Romans and later French poets. "Believe that every day is your last," says an exhortation from the third century poet Horace. Generations of visitors have scribbled their names on the plaques, anxious to immortalise themselves among the dead.

"Sometimes I'm all alone down here," says the man in galoshes from Pondicherry. "You can't be superstitious. If there were six million ghosts here, it wouldn't be possible to get through the door."

Water condenses on the ceiling and the steady "plunk, plunk, plunk" of dripping liquid is almost the only sound in the catacomb tunnels. From time to time you hear the distant rumble of a metro train - the living passing innocently by the dead. The gravel floor is soaked, and our shoes squelch as we walk through it.

The bones of Pascal and Montesquieu, Rabelais and La Fontaine, Robespierre and Danton are jumbled together here. Three tombstones commemorate the fatalities of French revolutionary battles. Jacobins and nobles, judges and their guillotined victims; they all lie here, skull against skull, bone upon bone.

The temperature rises as you climb towards the surface. Two guards stand by the turnstile. "We don't know why so many people come here," one says. "We ask ourselves all the time."

Maybe it's that old fascination with life beyond the grave, the thought that the Grim Reaper really will turn up for us one day. Up top, the smell of coffee wafts from cafes, and kiosques are selling thick weekend newspapers. But my shoes are coated with the beige-grey, slimy mud of the underworld.