Beginning this week is a brand new, four-part documentary series that examines the most astonishing and baffling UFO stories from around the UK. The opening instalment looks at an incident dubbed the ‘Welsh Roswell’. In 1974, locals near the Berwyn mountain range experienced an earthquake and reported seeing green lights in the sky. Some have claimed that a UFO crashed into the mountain and that the government has tried to cover up the truth.

Having held on to classified information about UFO sightings in Britain for over 30 years, the Ministry of Defence has now made the sensational decision to reveal all. In response to requests made under the Freedom of Information Act, the MoD plans to make available some 200 files detailing mysterious cases across the country since the 1970s. Starting this week, Britain’s Closest Encounters reveals the fascinating stories of these British ‘X-Files’.

One of the most famous British UFO cases took place in the Berwyn mountains. This remote area of Wales came to national attention one evening in January 1974, when a violent tremor measuring 3.2 on the Richter Scale shook the small town of Llandrillo. Locals were stunned to see strange lights in the sky, described by some as a “green arc” of light. “It was a very, very bright green light, different to anything I’d ever seen before,” recalls former police sergeant Elfed Roberts. “It was serious enough to get all the drinkers out of the local pub.”

Believing that an aircraft may have crashed into the mountain range, Roberts and his superior drove up the hill and searched the area by torchlight – but found nothing. Across the valley, on the opposite mountain, district nurse Pat Evans was conducting a similar search with her daughters. Evans reported seeing a perfect round circle hovering in the air, surrounded by smaller white lights, which then disappeared.

In the days to come, Llandrillo became the focus of media attention. Yet searches by RAF rescue teams, surveillance planes and a group of astronomers looking for evidence of meteorites found nothing. The Berwyn case slipped from people’s minds, until a UFO enthusiast interviewed Pat Evans in the 1980s and reignited interest. “Her story, when she gave it, resurrected the whole case,” says UFO investigator Jenny Randles.

The attention Evans received was so overwhelming that she eventually refused to discuss the matter and moved abroad. But her neighbour, Gerraint Edwards, has now decided to talk about his own sighting. For the first time in 34 years, Edwards reveals how he spotted a UFO in the Berwyn region three weeks after Evans’s sighting. “It looked like a rugger ball, but the ends of it were pointy,” he says. “When it took off, it just went like lightning on the same line as it hovered.”

Numerous theories have attempted to explain away the events in Berwyn of winter 1974. Jenny Randles has explored 10,000 UFO cases and is quick to point out that 95 per cent of them can be explained by terrestrial events. “In truth, very often in UFO phenomena, the solution is very much of this world,” she says. Some sceptics say the lights seen by Pat Evans were in fact those of the police searching on the opposite mountain. Still others suggest it was caused by meteor showers or light emitted from the ground following the earthquake.

Over time, the Berwyn mythology has grown, as rumours spread that the government was hiding the truth. Locals recall a military deployment in the wake of the earthquake that kept the Berwyn range out of bounds for weeks on end, while ‘men in black’ arrived to question people about what they had seen. One man even claimed to have been part of an operation to recover alien bodies from a crashed UFO. Sceptics say these claims are easily disproved, but what secrets do the newly released government files contain?