At the end of the eighteenth century, Paris was greatly overcrowded, flooded with far too many people who had come seeking work or adventure, for it was a fabled city in those days: a place of learning and elegance and fashion. But there was also a great deal of filth, disease and death, and the influx of new people, often bringing new diseases, filled the city cemeteries to overflowing. In areas such as Les Halles, the cemeteries were so overcrowded that proper burial was impossible. Corpses were flung down and covered with a shallow layer of earth which was swiftly covered by more corpses, until the ground of the cemetery bulged nearly twenty feet from the surrounding pavements. The whole neighbourhood stank with the charnel reek of the rotting dead, and the very air became diseased, creating yet more corpses. François Pontrain, the last gravedigger at the cemetery of Les Innocents, was said to have buried 90,000 bodies in 35 years. It became impossible for the people of Les Halles to keep milk, and wine spoiled in the casks. The sheer weight of all the corpses in Les Innocents caused the wall of a cave beneath it to collapse in 1780, destroying many homes and crushing the inhabitants. And so, in 1785, the State Council ordered that the cemetery of Les Innocents be destroyed, and converted into a public market.

It was unthinkable that the remains be destroyed, so they had to be moved. The cave which had collapsed beneath Les Innocents led into a whole system of chalk caves which spread for kilometres beneath Paris and beyond. Caves in the area known as the Tombe Issoire had been used before for burials: the area was consecrated, and some of the orders of the Knights Templar had tombs down there. It seemed a fitting place to store the remains from Les Innocents, so on April 7, 1786, the new ossuary was consecrated by the Vicar General of the Archbishop of Paris, and the transfer of bones began that very night. At nightfall, covered wagons for the poor, and funeral chariots and catafalques for the rich left from the cemetery for the ossuary site. The disinterment and removal took 15 months, but it went so successfully that it was decided to do the same with all the other parochial cemeteries, thus creating more land for buildings. Over the next seventy years the underground bonepiles stacked up until at the end of the nineteenth century nearly eight million corpses lined the caves under Denfert-Rochereau, and more ossuaries had to be created in caves at Montparnasse and Montrouge. Unofficial ossuaries also came into being as places to put 'problematic' corpses, such as those left over from the second wave of the French Revolution. With nearly 300km of tunnels beneath the city, there was still plenty of space.

Public visits to the ossuaries started in 1814 and were popular with thrill-seekers, but in 1830 the catacombs were closed to visitors, partly because of vandalism, and partly because they were still connected to the rest of the cave network, and many visitors, wandering off, got lost. When the catacombs were re-opened by Napoleon III most of the entrances to the cave system from the 'official' section of the catacombs at Denfert-Rochereau were sealed, and the caves shored up to make them safer for visitors. You can still tour some of this section: the entrance to the guided tours is opposite Denfert-Rochereau métro. It's dark, incredibly cold, and the ceilings are claustrophobically low. Water drips constantly down the walls in places, gradually fossilizing the skulls which line the walls, grinning at you with foul yellow teeth. Black chunks of human bone grind under your feet. The atmosphere, and the smell, of the remains of seven million dead Parisians is quite indescribable. For a look, take Kevin Kelm's virtual guided tour at (in English).

Of course unofficial visits to the caves and catacombs have a history as old as the caves themselves, although they've been a lot harder since 1995, when (supposedly) Algerian bomb threats led to the sealing of unofficial city entrances and a crackdown on unauthorised urban explorers. Access now is usually from outside the city, and explorers have to watch out for the tunnel police, who can spot-fine you huge sums. The tunnels are mostly unlit, fairly dangerous, and protective clothing is a good idea, but they're amazing: eighteenth-century revolutionary graffiti, discarded nameless bones, traces of habitation during several wars - and a feeling of exploration and discovery you don't get from the official tours. Some of the unofficial explorers, or ktaphiles, have detailed maps of the cave system. For more information and a good account of a visit through the 'real catacombs', visit (in English) There are a few other sites run by ktaphiles, mainly in French.