The police in Paris have made a curious discovery beneath the city.
A fully equipped subterranean cinema has been discovered below one of the city's museums.
Complete with an electric-powered screen and a bar, it was operated by what might be called an underground movement.
The French capital has hundreds of miles of tunnels, originally dug by the Romans quarrying for the stone to build the city.
In the 1700s, Paris started using some of the tunnels to store the bones of the dead - hundreds of thousands of them - as the medieval graveyards filled up.
The cinema was found beneath the Trocadero in Paris
Now, a new use has been discovered.
There is only one way to find out exactly what is going on down there - go down into the catacombs, via the one legal entrance to the catacombs on Place Denfert-Rochereau.
You have to go down 200 steps in a winding passageway - in the dark.
Once underground, it is damp, eerie and claustrophobic - the stone walls make you feel hemmed in, and the dripping ceiling is only inches above your head.
For me, it is hard to imagine coming down here for a night of fun, let alone opening up a cinema somewhere further on in these 300km (190 miles) of ancient tunnels.
But there is a group of people who love it down here - the "cataphiles", individuals who come down at night, in the dark, through manhole covers and via secret passageways, to make these underground tunnels and caves their own secret kingdom.
One of the artists and photographers who founded this underground movement is Patrick Alk, who says the discovery of the cinema is only one tiny example of what is actually going on beneath the Paris pavements.
"The group who created the cinema is one of many groups," he says.
"It all started in the 1980s. We were 500, maybe 1000 people, and we went under Paris to express ourselves as artists.
The tunnels beneath Paris are filled with bones (Photo: (c) Copyright 1996 John Kalucki)
"We did everything - from dance festivals, theatre, artists' happenings, exhibitions. We had a free kingdom underneath Paris, and the places there are really closed places, secret places."
Or at least they were until a recent police training exercise, when the Paris police stumbled upon the cinema.
Rather appropriately, it was located underneath the Trocadero complex, which includes the Museum of Cinema.
It was guarded by a camera, which set off a tape-recording of barking dogs to scare off intruders into this secret world. Inside were bottles of whisky, and copies of 1950s and 1960s "films noir".
When the police returned, there was a note from the cinephile cave-lovers saying: "Don't try to find us."
'Game with the police'
But in these security-conscious times, do the police consider the cataphiles a potential threat? Mr Alk says not.
"We are not considered a menace by the Paris police, because they prefer to have some people down there as a kind of control. If there were bad people down there, then we would know about it.
"With us, it is like a game with the Paris police. Relations with them are not so bad."
The Paris police do not quite put it that way, but their main concern is safety.
They advise Parisians and tourists not to venture into forbidden areas, in case they get lost, or the tunnels flood or collapse.
In 1993, one night-time reveller lost his way in the labyrinth. A plaque was put up to him by the other cataphiles after he was presumed dead.
That, of course, will not stop the truly dedicated cataphiles, who plan to continue their night-time revels.
And as for the mysterious cineastes, the police had contemplated charging them with the theft of electricity.
But in the end, the charge has been dropped - and presumably, those running it have now sought a more private spot elsewhere beneath the streets of Paris.