A 1974 "UFO incident" which was dubbed the Welsh Roswell was dismissed as an earthquake and a meteor combining, offical files show.
There was a huge bang and a brilliant light over the Berwyn Mountains in north east Wales, and there were later claims a spaceship crash was concealed.
Comparisons were drawn with Roswell, New Mexico, USA, in 1947, where it is claimed an alien crash was concealed.
But a Ministry of Defence (MoD) investigation said there was no UFO.
The National Archives files show it was explained by a noisy earth tremor coinciding with a meteor burning up in the atmosphere.
A search and rescue team was scrambled from RAF Valley on Anglesey, but found no wreckage on the mountainside.
The MoD investigation found that there were five other reports of UFOs seen over the UK at about 10pm on 23 January 1974, when the Berwyn Mountains incident happened.
Three sightings were in the Home Counties, one in Lincolnshire and another in Sussex.
Witnesses reported seeing a bright light in the north west which seemed to fall towards the horizon.
An expert who undertook independent research into the Berwyn Mountains incident for the British Astronomical Society reported that a "fireball" was visible over most of the UK that night.
Sightings were received from Somerset, Norfolk, Manchester and Edinburgh, the files notes.
The fireball descended from about 120km in the sky to about 35km before disintegrating over Manchester, the expert found.
Brynmor John, who was then junior RAF minister, explained the official position in a letter to Dafydd Elis Thomas, then a local MP, in May 1974.
Mr John wrote: "As suggested by the descriptions reported, it seems the phenomena could well have been caused by a meteor descending through the atmosphere burning up and finally disintegrating before it reached the ground.
"Such a hypothesis would also explain the absence of any signs of impact.
"It has also been suggested that at 8.32pm that evening there was an earth tremor in the Berwyn Mountains which produced a landslide with noises like detonation.
"The latter aspect is however outside the field of this department," Mr John added.
But the MoD's conclusions did not convince all those who witnessed the "Welsh Roswell".
The files also include a letter from one local who wrote: "That 'something" came down in the Berwyn Mountains on that night I am certain.
"It is certain to the minds of both my friends who came with me and to me that we were visited by an object that evening."