The cinema, with restaurant and bar annexe, was open for a seven-week season this summer, showing a suitably subversive programme which included works by Chinese and Korean directors but also Alex Proyas' Dark City, Coppola's Rumble Fish, David Lynch's Eraserhead, and Terry Gilliam's Brazil.

It was constructed in a series of interconnected caves totalling some 400 square metres beneath the Palais de Chaillot, across the Seine from the Eiffel tower. Former quarries, they were partly refurbished during the 1900 Universal Exposition when one of the galleries was clad with concrete to represent a future Channel tunnel and a wall was artfully terraced.

But the caves were sealed off for the last time at least 20 years ago and subsequently "ceased to exist officially", Lazar said. "We knew them well because we used them to get into the Palais de Chaillot every Bastille Day. The roof is the perfect place from which to watch the fireworks."

Indeed most of the LMDP's underground happenings are organised in places the city authorities are not aware of, he added. "There are so many underground networks - the quarries, the metro, the collective heating, the electricity, the sewers - and each is the responsibility of a different bureaucracy," he said.

"Urban explorers are the only people who, between us, know it all. We move between each network. We know where they link up - often, it's us who made the link. The authorities, the police, town hall, they don't know a hundredth, a thousandth, of what's down there."

Parts of the French capital are riddled with around 250 kilometres (150 miles) of underground tunnels, some of them dating from mediaeval times, and adepts who call themselves "cataphiles" are known to frequent them illegally and occasionally decorate galleries to hold parties or meetings.