UFO enthusiasts believe a UFO crashed in the Berwyn Mountains in Denbighshire and the bodies of the occupants taken to a military base in Wiltshire.
In January 1974, an earthquake measuring 3.5 on the Richter Scale shook the area then, soon after, mysterious glows were seen in the sky.
Now, a new witness reopens this mysterious case, and a documentary on it is to be shown on Channel 5 tonight....
New UFO witness reopens 1970s mystery of the 'Welsh Roswell'
01st July 2008
The Berwyn Mountains in Denbighshire, the site of an incident later dubbed "Britain's Roswell".
One of Britain's greatest UFO riddles refuses to go away as a new witness emerged today - 34 years after the alleged 'close encounter'.
The Government is said to have covered up 1974's event in North Wales, where scores of residents reported a massive tremor, strange lights in the sky and secret-service-style 'men in black' scouring the area.
It has been dubbed the 'Welsh Roswell' after the famous U.S. case in which aliens were allegedly found by authorities in New Mexico.
UFO believers claimed aliens crash-landed in the Berwyn mountain range and their bodies were transported by the MoD to top-secret Wiltshire research base Porton Down.
A scene from Channel Five documentary Britain's Closest Encounters reveals a flying saucer hovered for 10 minutes above the Berwyn Mountains in Denbighshire in 1974 before disappearing
No new details of the alleged incident emerged in May when hundreds of MoD documents about UFO sightings were released.
But now, fresh claims by retired gamekeeper Geraint Edwards, of Llandderfel, Denbighshire, have reopened the debate.
He told the makers of a new Channel Five documentary - being broadcast tonight - that a flying saucer hovered for 10 minutes above the mountains on February 15 1974 before it disappeared into space at impossible speed.
He said: 'It was definitely a flying saucer. It was a pity I didn't have a camera because it was there for at least 10 minutes, just hovering.
'We were on the way down to play darts when something caught our eye in the south-east, so we stopped.
'It looked like a rugger ball, but the ends of it were more pointy. When it took off, it just went like lightning on the same line as it hovered.
'I wrote it down in my diary. It was 6.45pm on the Friday night.
'If we were coming back from the pub, people would be saying, "They've had one or two." But we were going TO the pub.'
Three weeks before, on January 23, 1974, the villages of Llandrillo and Llandderfel, near Corwen, were rocked by a tremor measuring 3.5 on the Richter scale.
Reports of coloured lights and objects in the sky immediately afterwards, and unusual military activity in the following weeks, fuelled speculation that a UFO had crash-landed.
Sceptics maintain the explanation was an earthquake and a coincidental meteor shower.
The 'men in black' are explained as being seismologists researching the quake.
They also insist that a bright, twinkling moon-sized 'orb' seen by Mr Edwards's former neighbour Pat Evans was just a lamp carried by poachers on a nearby mountainside.
But one of the poachers has now told the Five documentary they had finished for the night by that time and their lights were switched off.
Farmer Huw Lloyd, 48, who was a teenager at the time, said: 'Whatever it was, it was kept quiet. I think there are things we should know about. And things that have happened have been covered up.'
Firefighter Adrian Roberts, who was thrown from his settee by the tremor in January 1974, told the programme makers: 'What are they hiding? There was a lot of military action in the area. Areas were secluded off from the public. It was about three months before anyone was allowed to go near the site.
'What people have seen and reported simply could not be made up.'
Retired North Wales Police Assistant Chief Constable Elfed Roberts, who was a sergeant at the time of the UFO incident, was rushing to Llandrillo moments after the tremor with his superior when they saw the mysterious lights.
He said: 'As we were driving, all of a sudden we saw this green light in the sky ahead of us and it seemed to be an arcing light, but it was very sudden, totally unexpected, different to anything ever seen before.'
• Britain's Closest Encounters is on Channel Five tonight at 8pm.