After entering the network through a drain next to the Trocadero, the officers came across a tarpaulin marked: Building site, No access.

Behind that, a tunnel held a desk and a closed-circuit TV camera set to automatically record images of anyone passing. The mechanism also triggered a tape of dogs barking, “clearly designed to frighten people off,” the spokesman said.

Further along, the tunnel opened into a vast 400 sq metre cave some 18m underground, “like an underground amphitheatre, with terraces cut into the rock and chairs”.

There the police found a full-sized cinema screen, projection equipment, and tapes of a wide variety of films, including 1950s film noir classics and more recent thrillers. None of the films were banned or even offensive, the spokesman said.

A smaller cave next door had been turned into an informal restaurant and bar. “There were bottles of whisky and other spirits behind a bar, tables and chairs, a pressure-cooker for making couscous,” the spokesman said.

“The whole thing ran off a professionally installed electricity system and there were at least three phone lines down there.”

Three days later, when the police returned accompanied by experts from the French electricity board to see where the power was coming from, the phone and electricity lines had been cut and a note was lying in the middle of the floor: “Do not,” it said, “try to find us.”