It's been called the "Welsh Roswell" but over 40 years on it's still not known exactly what happened in a remote part of North Wales on the night of Wednesday 23rd January 1974.
On an ordinary winter's evening in Wales, Merionethshire an isolated mountain range became the focal point of one of the UK's most famous UFO incidents.
Just after 8:30pm residents in Bala and the nearby villages of Corwen, Llandrillo and Llanderfel reported hearing something unusual, at least one, possibly two loud explosions.
The huge bang was followed by earth tremors and rumbling which lasted for around five seconds. One witness described the sounds as being "like a lorry running into a house."
The villagers, peering out of their windows and running out into the street to identify the source of this sound, looked up to the the Berwyn Mountains where they saw a mysterious white light above the mountains.
The local police force were inundated with calls from eye witnesses who believed the explosions and lights were the result of a UFO which had crashed into a side of Cadair Berwyn.
The official police logs mention "receiving 999 calls of UFO" from around 9pm on the night, it goes on to say "there's been a large explosion in the area and there is a large fire in the mountainside. I can see the fire where I am."
One local, a retired nurse, Pat Evans was concerned by the event and contacted Gwynedd Police. To start with she was unable to get through as their switch board was jammed with emergency calls but eventually she spoke to an officer who told her the disturbance was probably a plane crash.
Pat, along with her two daughters, jumped into the car and headed for the mountains, dreading what she might find but keen to offer any help she could. She said "we'd heard an almighty bang and we live at the foot of the mountain. I thought it might have been a plane crash. Me being a nurse and my girls being in St John Ambulance, I thought we could help."
As they drove along the B4391 towards the mountain, they climbed above the tree line and Pat saw a large pulsating sphere, glowing on the slopes of Cader Berwyn. She drove on for few minutes before turning around and returning to the same spot to observe the phenomenon.
Pat described the light as being a "perfect circle," she later told local press that "there were no flames shooting or anything like that. It was very uniform, round in shape."
The light was too far from the road to be reach on foot but Pat and her daughters watched as the sphere appeared to change colour several times, shifting from red to yellow to white, while smaller lights like fairy light danced around below it.
Due to its remote location, the object couldn't have got there by road, she said "it couldn't have got there any other way apart from being flown there, so it had to be a UFO of some sort. I'm talking about something that could only have got there by flying and landing."
Pat drove drove back to the village but along the way encountered police officers and soldiers who ordered her away from the mountain to allow them to close the road.
In the following days there was a large police and military presence in the area, the road remained closed and farmers were banned from working nearby land. Although searches were undertaken, the police say nothing was ever found on Cader Berwyn that night.
Could this really have been a crashed extraterrestrial craft in north east Wales? Well, the Ministry of Defence received five other UFO reports that same night, three from the Home Counties, one in Sussex and another in Lincolnshire.
Perhaps the most shocking claims from the Berwyn Mountains came several years after the incident when a UFO researcher not only confirmed the event did occur as the result of a crashed UFO crashed, but also claimed that alien bodies were found at the site.
Tony Dodd from 'UFO Magazine' claims that information was leaked to him by an anonymous military informant. The informant claims he was put on stand-by several days before the night of the alleged crash.
Soon after the event he and his unit comprising of four others soldiers were sent to Llanderfel, a village near the crash site, and ordered to collect "two large, oblong boxes" and deliver them to a government military science park in Wiltshire, called Porton Down.
The informant claims that upon arrival at the base the boxes were opened and they were shocked to see the dead bodies of two humanoid creatures, each about five to six feet tall and "so thin they looked almost skeletal." The informant went on to say the "the creatures were clearly not of this world."
According to Dodd his informant later rejoined other members of his unit who said they had also transported alien bodies to Porton Down but say their cargo had still been alive.
Comparisons were immediately drawn to the famous Roswell Incident in New Mexico, 1947, one newspapers labelled the event "The Roswelsh Incident."
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A lot of the witnesses of the events which took place on 23rd January 1974 have been misquoted, had their reports taken out of context or simply didn't exist, their stories being fabricated.
For example, Pat Evans is said to be furious over claims that she was stopped on the night by military personnel who were cordoning off the B4391. This claim originated from a misunderstanding when she was first interviewed by a UFO researcher but in interviews since she's quoted as saying, "we didn't see anybody on the road even though there were various reports that we were told to go away by armed police and military etc, which was all totally untrue."
Claims of a high police and military presence in the area on the night and following the incident also seem to be untrue and originate from the reports of just one article in the Border Counties Advertiser. In fact the police logs show that just three officers were sent to investigate the strange lights.
But if the explosion and lights weren't the result of a UFO crash, or even a plane crash, then what was it? In 2010 the MOD released a report which states that the event was a rare combination of an earthquake combined with sightings of an unusually bright meteor.
According to the British Geological Survey the tremor had a recorded magnitude of 3.5 on the Richter scale and was felt as far away as Liverpool, emanating from an epicentre at Bala Lake in the Berwyn Mountains.
Records kept at the Astronomy Department at Leicester University, report that several unusually bright meteors were seen over the area that night. Known as "bolide meteors" these differ from normal shooting stars as they are much brighter and longer lasting as they burn up in the atmosphere.
But that still doesn't explain what Pat and her daughters saw on the hillside. It seems that while Pat was driving along the mountain road the police who were sent to investigate what they believed could be a plane crash were making their way up the mountain in a Landrover they'd commandeer from a local farm.
Part way up the mountain the police were forced to move a car belonging to known, local poachers who were also on the mountainside that night. Once past the obstruction the police searched the area by torch light.
The timings suggest it's highly likely that what Pat witnessed was the powerful light used by the poachers which was powered by a car battery and what she described as "fairy lights" moving around below the light were probably the torches of the searching police officers.
The MOD reports confirmed there was no UFO crash in North Wales that night and to this day the official web page for Porton Down says, "no aliens, either alive or dead have ever been taken to Porton Down."