We know this because France's national space agency (CNES) has just become the first official body to open its UFO files to the public. Its motivation is twofold - to publicise a number of unexplained cases to the scientific community, and to undermine those conspiracy theorists who view secrecy as proof of, well, secrets.
Many of the sightings documented over the past five decades have been fully explained. Some were found to be pieces of rocket debris falling back to earth, others hoaxes. In one case, a bright, burning object turned out to be the spontaneous combustion of buried German ordnance from the second world war. But CNES admits 28% of its X-files "remain unexplained".
In 1967, for example, two children tending the family herd in the village of Cussac, central France, spotted "four small black beings", who rose into the air and entered the top of a spherical object, then returned to the cow field to collect something and finally zoomed off into space leaving behind unusually dry grass and an odour of sulphur. In January 1981, a witness claims to have seen a saucer-shaped object land in the village of Trans-en-Provence. Analysis showed that the ground around the scene had been heated up, that the object weighed several hundred kilos, and that surrounding plants "underwent biological changes". One of the most detailed sightings came from the pilots of an Air France jet in 1994. All three witnessed a large, reddish-brown disc hovering in the sky above Paris. Radar evidence appeared to confirm the sighting.
In Britain, UFO files can only be requested from the Ministry of Defence on a case-by-case basis under the Freedom of Information Act. Roy Lake, chairman of London UFO Studies, says we have a right to know everything. "The French have done the right thing. Until now governments haven't wanted to be open about UFO sightings because they don't want to admit they can't do anything about them - that they're as helpless as the rest of us."