There is an obscure door in a Paris suburb which opens onto mile upon mile of tunnels and catacombs - the city of eternal darkness
Right in the centre of Paris, hewn into solid rock, is a splendid replica of Menorca's Port Mahon prison. In the late 18th century, Francois Decure, a quarry worker who had been imprisoned in the Mediterranean fortress, spent five long years chipping away at this masterpiece. But finding Decure's sculpture isn't as easy as you would think. For it lies more than 20 metres below the busy Place Denfert-Rochereau, in the depths of the city's maze of ancient quarries.

Beneath the elegant surface, Paris is not unlike a piece of Swiss cheese. No other city is so riddled with tunnels and holes, many of which date back centuries. The quarries meander deep below the ground for an astounding 300km, and the portion known as the "catacombs" now constitutes the largest ossuary on earth.

The official gateway to this fascinating place - there are apparently at least 200 other clandestine entrances - is located in the centre of Place Denfert-Rochereau, a boisterous intersection in the 14th arrondissement. It is here that the guided trips start.

Along with a group of visitors, I descended a steep spiral staircase to embark on a two-hour trek. We walked briskly along the first 300m of sparsely lit tunnels, their crude rock ceilings looming low and heavy overhead. Apart from the faint dripping of groundwater and the crunch of our footsteps on the gravelly path, all was quiet.

Local people had been building with the area's gypsum and limestone from around the time of the early Gallo-Romans, with most of the quarrying taking place in what is now the 13th, 14th and 15th arrondissements. Rainwater seeping through the porous rock helped to carve out huge, cathedral-like spaces, leaving very fragile surface layers; by the late 18th century, not surprisingly, some dramatic cave-ins occured. Only then, as whole houses were suddenly swallowed up, were the consequences of the quarrying addressed.

In 1777, Louis XVI created the Inspection Generale des Carrieres (Quarries Inspection Service). Its mission was to document and map out the 835 hectares of underground spaces and consolidate them - and to put them to good use. Paris's graveyards were overflowing to the extent that the stacked remains of the Innocents Cemetery, in the present-day Les Halles district, reached two metres above street level.

Beginning in 1785, bones were transferred to a large quarry near what is now the Place Denfert-Rochereau. Day after day, workers carted them to the quarry to drop them down shafts. At the bottom, they were sorted and arranged methodically. The closure of other Parisian graveyards soon followed, and their contents were also moved to the new catacombs. Today, the skeletons of a phenomenal six million former Parisians rest in a series of galleries known as the ossuary; despite the name, the site is not a true catacomb, since there are no religious rites attached to it.

In the damp and narrow corridor leading to this underground necropolis stands Francois Decure's sculpture and, nearby, the workers' "baths". A staircase carved into the rock leads down to a clear pool at the level of the water table. This is where the quarry workers would refresh themselves before heading back up to the surface.

A sign at the ossuary entrance warns the faint-hearted: "Arretez! - c'est ici l'empire de la mort" (Halt! This is the empire of death). Beyond lies a mind-boggling sea of human skulls, tibias and other fragments piled from floor to ceiling, some up to 1,200 years old. There are several distinct chambers spread over 11,000 square metres. Among the indistinguishable remains piled up here are the bones of the philosophers Pascal and Montesquieu, the writers Rabelais and Madame de Sevigne, the alchemist Nicolas Flamel and the politician Mirabeau. In one of the last chambers is "La Rotonde des Tibias", a thick, circle composed of hundreds of tibias.

Beyond the ossuary are larger, bell-shaped galleries, where faint rays of light penetrate from openings at the surface. These voluminous spaces have served, among a variety of functions, as bomb shelters and, perhaps most extraordinarily, as the mushroom farms that, from 1812 until recently, produced the "champignon de Paris".

From time to time new ideas are put forth as to possible uses: discotheques, sports centres and hotel complexes have all been proposed. In the surrounding suburbs, where another 1,300 hectares of quarries exist, some such projects have actually been carried out.

Enormous though they are, however, the quarries are only part of Paris's underground scene. The city also boasts a 2,100km of vaulted sewers, and hundreds more of metro and train tunnels. So interconnected is the network that, on 18 August 1944, the command centre of the Parisian resistance was actually set up under the Place Denfert Rochereau. From there, a strategic plan was laid out for the utilisation of the sewer, the quarries and a part of the Metro.

Having covered nearly two kilometres in our subterranean journey, we made our way along the final corridor, beneath the rue Dareau, to a point of exit. A dusty staircase leads us back to the surface and out, through an obscure door, into the dry summer heat.

As we step back into the "City of Light", amazed but somewhat relieved to leave behind that forgotten zone of darkness, I can't help but think of the unfortunate Philibert Aspairt, one-time porter at the nearby Val- de-Grace Hospital. In 1775 he gave in to curiosity and descended into the quarries. Alone, he got lost, and 11 years passed before his skeleton was eventually found by quarry inspectors.

Thankfully, the galleries are patrolled more regularly today, though 18th-century visits were fraught with danger. After completing his sculpture, Decure was crushed to death while trying to create easier access to his work.

The entrance to Les Catacombes is at 2, Place Denfert-Rochereau, 75014 Paris (00 33 1 43 22 47 63). Visits take place between Tuesday and Friday from 4pm to 6pm and on Saturdays and Sundays from 9am to 11am (closed on public holidays). The nearest Metro is Denfert-Rochereau. Remember to bring a flashlight

Traveller's Guide

THE ONLY sensible way to get to Paris from southern England this summer is on Eurostar (0990 186186). Families with youngsters are being offered the chance to take their children free - and, for the purposes of this deal, "children" are defined as being under 16, rather than the normal under-12s. Each adult pays pounds 79 for a return, and can take two children. In theory, therefore, two parents with four children aged between 12 and 15 can save more than pounds 300 on a return trip.

From other parts of Britain, the absence of the promised "Regional Eurostar" trains gives the airlines a definite edge. The lowest fares are likely to be on Ryanair (0541 569569), from Prestwick in southwest Scotland to Beauvais, a little north of the capital.

On Channel crossings, the end of duty-free has not brought the predicted hike in fares. For example, Hoverspeed (08705 240241) has a five-day fare of pounds 137 return for a car plus two passengers - even in peak season around the eclipse on 11 August - on its Folkestone- Boulogne and Newhaven-Dieppe ferries.

More information: Contact the French Travel Centre, 178 Piccadilly, London W1V 0AL (0891 244123).