By day, Lazar Kunstmann is a typically avant-garde Parisian, an urbane, well-spoken video film editor who hangs out in the fashionable Latin Quarter. By night he inhabits a strange and secret world with its base in the tunnels beneath the French capital – the world of the urban explorers.

Mr Kunstmann belongs to les UX, a clandestine network that is on a mission to discover and exploit the city’s neglected underworld. The urban explorers put on film shows in underground galleries, restore medieval crypts and break into monuments after dark to organise plays and readings. In the eyes of their supporters, they are the white knights of modern culture, renovating forgotten buildings and staging artistic events beyond the reach of a stifling civil service.

The authorities view them differently: as the dark side of the City of Light – irresponsible, paranoid subversives whose actions could serve as a model for terrorists. A police unit has been trained to track les UX through the sewers, catacombs and old quarries that are their pathways under Paris. Prosecutors have been instructed to file charges whenever feasible.

The stand-off is symbolic of French society: a rigorous bureaucracy on the surface with a bizarre subculture below.

Mr Kunstmann, a spokesman for the movement, met The Times last week in the back room of a bar in central Paris. Beside him sat a thin, austere-looking woman who sipped a beer, gave her name only as Lanso and barely said a word throughout the interview.

From time to time, however, she whispered into Mr Kunstmann’s ear and he relayed the message. “We are the counterpoint to an era where everything is slow and complicated,” he said. “It’s very difficult to get anything done through official channels. If you want to do it, you have to be clandestine.”

Mr Kunstmann said that les UX had 150 or so members divided into about ten branches.One group, which is all-female, specialises in “infiltration” – getting into museums after hours, finding a way through underground electric or gas networks and shutting down alarms. Another runs an internal message system and a coded, digital radio network accessible only to members.

A third group provides a database, a fourth organises subterranean shows and a fifth takes photographs of them. Mr Kunstmann refused to talk about the other groups.

He did, however, say that Lanso was the leader of a branch called the Untergunther – the name comes from a German record whose music served as an alarm on an early mission – which specialised in restoration. This group, whose members include architects and historians, rebuilt an abandoned 100-year-old French government bunker and renovated a 12th-century crypt, he said. They claim to be motivated by a desire to preserve Paris’s heritage.

Last year the Untergunther spent months hidden in the Panthéon, the Parisian mausoleum that holds France’s greatest citizens, where they repaired a clock that had been left to rust. Slipping in at closing time every evening – French television said that they had their own set of keys – they set up a workshop hidden behind mock wooden crates at the top of the monument. The security guards never found it. The Untergunther used a professional clockmaker, Jean-Baptiste Viot, to mend the 150-year-old mechanism.

When the clock began working again, officials were horrified. The Centre for National Monuments confirmed that the clock had been repaired but said that the authority had begun legal action against the Untergunther. Under official investigation for breaking and entry, its members face a maximum sentence of one year in prison and a €15,000 (£10,500) fine.

“We could go down in legal history as the first people ever to be prosecuted for repairing a clock,” said Mr Kunstmann. But he was unrepentant.

“In any other country, a monument such as the Panthéon would be maintained in a perfect state. But not in France. Here, if we hadn’t restored the clock, no one else would have bothered.”

Cities down under

— The earliest inhabitants of Cappadocia, Turkey, dug cavitites into the volcanic rock and linked them with tunnels to escape from wild animals and harsh weather. Gaziemir, discovered this year, includes two churches and a Turkish bath

— Burlington is a subterranean city 100ft (30m) beneath Corsham, Wiltshire. Built in the 1950s as the Emergency Government War Headquarters, it has 60 miles (97km) of road, a BBC studio and a pub

— In 1999, a 2,000-year-old underground complex was discovered on the island of Kish, 11 miles off the coast of Iran. It was developed to house shops, restaurants and art galleries

— Thirty-five per cent of all commercial space in downtown Montreal, Canada, is in the world’s largest underground complex. About 500,000 people use it every day to navigate the city

— The undergound heating system of Ulan Bator, the Mongolian capital, has become home to more than 4,000 street children

Sources: www.aksaray.gov.tr; www.corshamtown.co.uk; www.vieux.montreal.qc.ca; www.mediarights.org

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