Exactly 60 years ago, a light aircraft was flying over the Cascade Mountains in Washington State, at a height of around 10,000ft.

Suddenly, a brilliant flash of light illuminated the aircraft. Visibility was good and as pilot Kenneth Arnold scanned the sky to find the source of the light, he saw a group of nine shiny metallic objects flying in formation.

He estimated their speed as being around 1,600 miles per hour - nearly three times faster than the top speed of any jet aircraft at the time. He described the craft as arrow-shaped and said they moved in a jerky motion - 'like a saucer would if you skipped it across the water'.

A reporter seized on this phrase and in his story described the objects as 'flying saucers'. The age of the Unidentified Flying Object (UFO) had begun.

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Soon, similar reports began to come in from all over America. This wasn't just the world's first UFO sighting, this was the birth of a phenomenon, one that still exercises an extraordinary fascination.

Then, two weeks after Arnold's sighting, something happened that was to lead to the biggest UFO conspiracy theory of all time. On or around July 2, 1947, something crashed in the desert near a military base at Roswell, New Mexico.

Military authorities issued a press release, which began: "The many rumours regarding the flying disc became a reality yesterday when the intelligence officer of the 509th Bomb Group of the Eighth Air Force, Roswell Army Air Field, was fortunate enough to gain possession of a disc."

The headlines screamed: 'Flying Disc captured by Air Force.' Yet, just 24 hours later, the military changed their story and claimed the object they'd first thought was a 'flying disc' was a weather balloon that had crashed on a nearby ranch.

Amazingly, the media and the public accepted the explanation without question, in a way that would not happen now. Roswell disappeared from the news until the late Seventies, when some of the military involved began to speak out.

The key witness was Major Jesse Marcel, the intelligence officer who had gone to the ranch to recover the wreckage. He described the metal as being wafer thin but incredibly tough.

It was as light as balsa wood, but couldn't be cut or burned. Some witnesses described seeing strange inscriptions on the wreckage.

These and similar accounts of the incident have largely been dismissed by all except the most dedicated believers.

But last week came an astonishing new twist to the Roswell mystery - which casts new light on the incident and raises the possibility that we have, indeed, been visited by aliens.

Lieutenant Walter Haut was the public relations officer at the base in 1947, and was the man who issued the original and subsequent press releases after the crash on the orders of the base commander, Colonel William Blanchard.

Haut died last year, but left a sworn affidavit to be opened only after his death.

Last week, the text was released and asserts that the weather balloon claim was a cover story, and that the real object had been recovered by the military and stored in a hangar. He described seeing not just the craft, but alien bodies.

He wasn't the first Roswell witness to talk about bodies. Local undertaker Glenn Dennis had long claimed that he was contacted by authorities at Roswell shortly after the crash and asked to provide a number of child-sized coffins.

When he arrived at the base, he was apparently told by a nurse (who later disappeared) that a UFO had crashed and that small humanoid extraterrestrials had been recovered. But Haut is the only one of the original participants to claim to have seen alien bodies.

Haut's affidavit talks about a high-level meeting he attended with base commander Col William Blanchard and the Commander of the Eighth Army Air Force, Gen Roger Ramey. Haut states that at this meeting, pieces of wreckage were handed around for participants to touch, with nobody able to identify the material.

He says the press release was issued because locals were already aware of the crash site, but in fact there had been a second crash site, where more debris from the craft had fallen. The plan was that an announcement acknowledging the first site, which had been discovered by a rancher, would divert attention from the second and more important location.

Haut also spoke about a clean-up operation, where for months afterwards military personnel scoured both crash sites searching for all remaining pieces of debris, removing them and erasing all signs that anything unusual had occurred.

This ties in with claims made by locals that debris collected as souvenirs was seized by the military.

Haut then tells how Colonel Blanchard took him to 'Building 84' - one of the hangars at Roswell - and showed him the craft itself. He describes a metallic egg-shaped object around 12-15ft in length and around 6ft wide. He said he saw no windows, wings, tail, landing gear or any other feature.

He saw two bodies on the floor, partially covered by a tarpaulin. They are described in his statement as about 4ft tall, with disproportionately large heads. Towards the end of the affidavit, Haut concludes: "I am convinced that what I personally observed was some kind of craft and its crew from outer space."

What's particularly interesting about Walter Haut is that in the many interviews he gave before his death, he played down his role and made no such claims. Had he been seeking publicity, he would surely have spoken about the craft and the bodies.

Did he fear ridicule, or was the affidavit a sort of deathbed confession from someone who had been part of a cover-up, but who had stayed loyal to the end?

Another military witness who claimed to know that the Roswell incident involved the crash of an alien spacecraft is Colonel Philip J. Corso, a former Pentagon official who claimed his job was to pass technology from the craft recovered at Roswell to American companies.

He claims that discoveries such as Kevlar body armour, stealth technology, night vision goggles, lasers and the integrated circuit chip all have their roots in alien technology from the Roswell crash.

Corso died of a heart attack shortly after making these claims, prompting a fresh round of conspiracy theories.

As bizarre as Corso's story sounds, it has support from a number of unlikely sources, including former Canadian Minister of Defence Paul Hellyer, who spoke out recently to say that he'd checked the story with a senior figure in the U.S. military who confirmed it was true.

The U.S. government came under huge pressure on Roswell in the Nineties. In July 1994, in response to an inquiry from the General Accounting Office, the Office of the Secretary of the Air Force published a report, The Roswell Report: Fact vs. Fiction in the New Mexico Desert.

The report concluded that the Roswell incident had been attributable to something called Project Mogul, a top secret project using high-altitude balloons to carry sensor equipment into the upper atmosphere, listening for evidence of Soviet nuclear tests.

The statements concerning a crashed weather balloon had been a cover story, they admitted, but not to hide the truth about extraterrestrials.

A second U.S. Air Force report, The Roswell Report: Case Closed, was published in 1997 and focused on allegations that alien bodies were recovered.

It concluded that any claims that weren't entirely fraudulent were generated by people having seen crash test dummies that were dropped from balloons from high altitude as part of Project High Dive - a study aimed at developing safe procedures for pilots or astronauts having to jump from extreme altitudes.

These tests ran from 1954 to 1959 in New Mexico, and the U.S. government suggested that sightings of these dummies might have been the root of stories about humanoid aliens, with people mistaking the dates after so many years, and erroneously linking what they'd seen with the 1947 story of a UFO crash.

Sceptics, of course, will dismiss the testimony left by Haut. After all, fascinating though it is, it's just a story. There's no proof. But if nothing else, this latest revelation shows that, 60 years on, this mystery endures.

UFO enthusiasts plan to commemorate the 60th anniversary of the Roswell incident with a series of events. In Roswell itself there will be a conference partly sponsored by the city authorities. Thousands are predicted to attend. Roswell has become not just big news, but big business.

Ever since Kenneth Arnold's sighting and the Roswell incident, UFO sightings have continued to be made around the world.

In the UK, in 1950, the Ministry of Defence's Chief Scientific Adviser, Sir Henry Tizard, said UFO sightings shouldn't be dismissed without proper, scientific investigation.

The MoD set up arguably the most wonderfully named body in the history of the Civil Service, the Flying Saucer Working Party. Its conclusions were sceptical.

It believed UFO sightings were attributable to either misidentifications, hoaxes or delusions. Its final report, dated June 1951, said no further resources should be devoted to investigating UFOs.

But in 1952 a high-profile series of UFO sightings occurred, in which objects were tracked on radar and seen by RAF pilots. The MoD was forced to think again and has had been investigating ever since. To date, the MoD has received more than 10,000 reports.

The best-known UK incident occurred in December 1980 in Rendlesham Forest, Suffolk. In the early hours of December 26, personnel at RAF Bentwaters (a base leased to the USAF) reported strange lights in the forest. Thinking an aircraft had crashed, they went to investigate.

What they found, witnesses say, was a UFO. They took photographs (which they were later told hadn't come out) of the brightly illuminated craft and one of the men got close enough to touch the object, which then took off and flew away. The stunned men briefed their bosses, including the deputy base commander, Lieutenant Colonel Charles Halt.

Halt ordered the men to make official witness statements, including sketches of the craft. The following night Halt was at a social function when a flustered airman burst in, saluted and said: "Sir, it's back."

Halt looked confused and said: "What's back?" "The UFO, Sir. The UFO is back," the airman replied.

Halt and a small team went to investigate. His intention, he later reported, was to 'debunk this nonsense'. As they went into the forest, their radios began to malfunction and powerful mobile searchlights cut out. Suddenly, Halt and his team saw the UFO and attempted to get closer. At one point it was directly overhead, shining a bright beam of light down on them.

After these events, Halt ordered an examination of the area where the UFO had been seen on the first night. Three indentations were found in the ground where the craft had landed. A Geiger counter was used and radiation readings were taken, which peaked in the three holes. Halt reported it to the MoD and an investigation began.

This was inconclusive, but Defence Intelligence Staff assessed the radiation readings taken at the landing site were 'significantly higher than the average background'. The MoD's case file on the incident has only recently been released under the Freedom of Information Act.

Another spectacular UFO incident occurred in March 1993. Over six hours, around 60 witnesses in different parts of the UK reported a series of sightings of spectacular UFOs. Many of the witnesses were police officers and the UFO also flew over two military bases in the Midlands, RAF Cosford and RAF Shawbury.

The Meteorological Officer at RAF Shawbury described the UFO as being a vast triangular-shaped craft that moved from a hover to a speed several times faster than an RAF jet in seconds.

He estimated that the UFO was midway in size between a Hercules transport aircraft and a Boeing 747 and said that at one point the craft had been as low as 400ft. He also said that it had been firing a narrow beam of light at the ground and emitting an unpleasant low-frequency hum.

The MoD investigation lasted several weeks and the case file - also recently released - runs to more than 100 pages.

The final briefing submitted to the Assistant Chief of the Air Staff stated: "In summary, there would seem to be some evidence on this occasion that an unidentified object (or objects) of unknown origin was operating over the UK." That is about the most frank admission on UFOs that the MoD has ever made.

Sixty years after Kenneth Arnold's 'flying saucer' sighting, pilots are still seeing UFOs. In April this year, Captain Ray Bowyer, a pilot based in Alderney, saw two bright yellow UFOs in the vicinity of the Channel Islands.

Some of his passengers saw the same thing, another pilot in the area made a similar report and some unusual readings were seen on air traffic control radar. The MoD and the Civil Aviation Authority investigated the incident and no explanation has been found.

Despite any number of hoaxes over the years, interest and belief in UFOs remains strong. Under the Freedom of Information Act, the MoD receives more requests relating to UFOs than on any other subject.

So what is it about UFOs that continues to excite our imaginations? To some people, the subject has become almost a religion and perhaps that gets to the heart of it. Those who study the subject are on a quest not just for the truth, but for meaning. It's a search for the answer to one of the most fundamental questions we can ask - are we alone?