UFO watchers believe that in 1947 a flying saucer with aliens on board landed outside the New Mexico town of Roswell and that an elaborate cover-up by the authorities followed. The BBC's Kevin Connolly went to Roswell in pursuit of the truth about the Roswell incident.
There is a lunar quality to the landscape of New Mexico which seems somehow appropriate for a state which is our portal to the heavens.
It is here on a dried-up lake bed high above sea level that the radio telescopes of the US government's Very Large Array (VLA) receive signals from the outer edges of our expanding universe, chasing the very moment of the Big Bang through the trackless void of time and space.
And of course it is also here - perhaps - that 62 years ago a flying-saucer crashed to earth on a ranch outside the town of Roswell, killing its alien crew and prompting one of the most elaborate and protracted cover-ups in history.
The power of that possibility and the darkness of the nights here so far from the light pollution of the big cities are what draw scientists and curious tourists alike to this entrancing place.
And it is what motivates watchers of the skies to keep, well, watching the skies, obviously.
If UFO true-believers are right, then nothing much that has happened on our tiny, fragile planet in the years since that stormy summer's night really matters very much.
What, after all, would Watergate, or Vietnam or Iraq amount to if we could establish that the US government knew for sure that we are not alone in the universe?
Most of the big questions about alien life and UFOs can be traced back to Roswell - not least the issue of how life-forms from another civilisation have such an uncanny sense of when the tourist industry in a small US town could use a shot in the arm.
Are we really alone in the heavens, for example, and if we are not, do the civilisations with which we share the heavens mean us any harm?
Is it just a coincidence that aliens have never managed to find an earth-dweller who knows how to operate his own camera properly?
And why, if you have journeyed light years across the unknowable vastness of the heavens, would you confine yourself to a fleeting and ambiguous appearance before a handful of New Mexican ranchers?
Why not go the extra mile and find a research institute of some kind - unless our visitors have a sense of humour, of course.
But we are getting ahead of ourselves.
Let us return to that stormy summer New Mexican night in 1947 when the story of the UFO landing first broke.
It was a world of tension and uncertainty.
The United States had detonated the first atom bombs - they were developed just up the road at Los Alamos, New Mexico - and was uneasily aware that the Soviet Union, its increasingly hostile former ally, had nuclear ambitions of its own. The Cold War was just beginning.
Roswell was in those days the home base of the 509th Bombardment Group of the US Eighth Army Air Force.
Most of the boys in the 509th were combat veterans and when they were tasked to investigate reports of some kind of landing on a ranch a short distance away it seems reasonable to assume they were not too excited at first. That soon changed.
When they got the material back to base, they quickly concluded they were onto something historic.
Their first press release talked of the recovery of a flying saucer - it was only when the suits descended from Washington that the tone of the official communiques changed.
The base intelligence officer who was tasked with taking the wreckage to another base reports leaving it in an office there and returning a few minutes later to find that the space debris he had brought had been replaced with parts of a weather balloon.
The fix was in. Faced with evidence of one of the most significant events in human history the American authorities had responded by pulling the old switcheroo.
One account of those days comes from Julie Shuster, whose father was the press officer at the Roswell base.
She now runs the museum in the town which is the focal point for the local UFO industry (it is on a street where the street lights have been decorated so that they look like alien heads).
For Julie there is a simple issue in all of this which goes back to the version of events her father passed on to her.
"My daddy didn't lie. My father saw the bodies, my father saw the craft," she says.
"He saw bodies - large heads, almond shaped eyes... and material that couldn't be burnt, ripped, cut - anything."
It is not quite so personal for the other true believers in the incident.
Dennis Balthaser, for example, is a retired civil engineer who is perhaps the most meticulous researcher of the Roswell incident.
He has spent years (and thousands of dollars of his own money) tracing every witness and every player from that night in 1947 and is convinced that there was a landing.
But he is at his most compelling when talking about the cover-up which follows.
Dennis lives in a rather frightening world where the US government would be perfectly happy to murder anyone (including him) who got too close to the truth.
He believes the US is really governed by a kind of secret committee of senior military and intelligence officials with the president serving as a kind of hired hand to deal with the public.
"Basically he can't be trusted with information like this" says Dennis.
"Back in Truman's time, we're looking at a thing called 'majestic 12', which was a group of some of the highest military people, some of the highest dignitary people we had. I believe today we still have a group similar to that that calls the shots."
Even before I met Dennis, I knew he believed that space travellers helped build the pyramids - where I, for example, am more inclined to the view that they are probably the work of Egyptians.
I expected to find him hopelessly naive - but the funny thing is he is so lucid and convincing that I left feeling rather naive myself.
(And in case you were wondering - if anyone ever lifts a quote from this article to promote a book or a DVD it will be that previous sentence.)
But here is the problem.
I mentioned before that New Mexico is a place where real science and Roswell science-lite co-exist.
And it is in that proximity that our explanation probably lies.
At the VLA (that centre of government radio telescopy) they will tell you that around the time of the crash, the US government was sending up special high-altitude weather balloons made of a then-classified material.
They were looking for atmospheric evidence that the Russians were testing their own nuclear bomb.
On the night of the Roswell incident, one of those balloons went missing.
Until, perhaps, it was found by the boys of the 509th.
And all that stuff about alien bodies being recovered and autopsies being performed? Well I leave you to speculate about that for yourselves.
If you find all that a little disappointing then you can always employ the debating technique favoured by UFO true believers - the deployment of questions designed to expose the lack of absolute certainty in almost all human affairs.
How do you know, for example, that there really is not a government department 27 layers above top secret tasked with keeping an eye on these things?
Perhaps they insist that articles like this are submitted to them for screening before publication.
And maybe this is not the script I originally wrote and submitted - just the script they sent back.
You could hardly blame me for giving in - I am sure you would not want to find my bleached bones left out in the desert, would you?
All I am saying is - keep watching the skies.