Crop circles have baffled sceptics and believers for 20 years, and now feature in a Hollywood blockbuster. Nick Morrison looks at a phenomenon still resisting attempts at explanation.

THEY appear overnight, without warning and seemingly leaving no trace of their origins, as sudden as they are mysterious. Some are plain circles, others a string of intricate formations, but, whatever their shape, they are a source of fascination to all but the farmers whose crops have been flattened in the process.

Like the lost city of Atlantis, the Loch Ness Monster, the Easter Island statues and the building of the pyramids, crop circles have become one of the Earth's great unsolved riddles. Although they have come to prominence only in the last 20 years, crop circles have their origins in the distant past, with the UK acknowledged as the world centre of crop circle activity.

Now the Hollywood film Signs - which sees Mel Gibson confronted by mysterious patterns in his cornfield - has brought a renewed burst of interest in the phenomenon, as well as bringing visitors across the Atlantic to try and witness it at first hand.

Theories behind the formations have ranged from warnings from extra terrestrials, the result of the Earth's magnetic field, secret military experiments, frantically rutting hedgehogs, or two men with a plank of wood and a rope. Only the last of those is known to have been responsible for at least some of the circles. But there are still those who refuse to accept such a mundane explanation.

The earliest known formation was in England in 1647, with evidence surviving in the form of a woodcarving, but it was not until the late 1970s that crop circles began appearing in any significant number. The early days saw simple circles, with the variation confined to swirling the crops either clockwise or anti-clockwise. But later versions became more elaborate, adding straight lines, spirals and complex patterns.

Although they have appeared all over the world, most of the more intricate formations have been found in the UK, leading researchers to conclude that we are living in a hotbed of paranormal activity. Either that, or we have more than our fair share of practical jokers.

But it is not just the formations themselves which have excited investigators. People have reported feeling nauseous after stepping into a crop circle, only returning to normal after leaving its boundaries. Others claimed a sudden and inexplicable relief from the symptoms of conditions including arthritis or osteoporosis after exposure to the mysterious forces.

Some farmers claim to have had greater yield from their crops in the years after formations appeared in their fields, and there have been reports of electronic equipment failing in the vicinity of crop circles. There are also those who can hear a "trilling" noise coming from the crops affected.

Colin Andrews, one of the world's foremost experts on the phenomenon, has laid out a number of basic theories for the origin of crop circles. One is that they are the result of whirlwinds, but while this may explain the circles of the 1970s, it seems to fall down as far as the more complex patterns are concerned.

Other theories, including that they are the work of extra terrestrials, the result of the Earth's gravitational field or connected to archeological features below ground, have proved harder to doubt. As well as impossible to prove.

The believers suffered a bit of a setback in 1991, when Doug Bower and Dave Chorley put their hands up as the masterminds behind the circles. While Dave stood on one end of a five foot iron bar, Doug pulled it around in a circle, bending the corn as he went.

When meteorologists came up with the whirlwind explanation, the pair graduated to more elaborate designs, with Doug sketching them out in a workshop, before taking a plank of wood and a rope to a cornfield. After walking around with the rope in ever-decreasing circles, Doug created a passage to another circle, using a sight dangling from a baseball cap to keep the line straight.

But even after they went public, there were some who refused to believe them, and carried on looking for visitors from outer space, citing the unusual angles at which the crops were bent, or that some stalks were bent and not broken, or that the patterns were too precise and too complicated for human hands to have created, as evidence there was no hoax.

Others claiming to have created circles have been chased and threatened by angry believers, who fear the phenomenon is being devalued, with accusations flying of the secret service trying to distract attention from the genuine formations made by the aliens.

Colin Andrews acknowledges that some of the circles are hoaxes, but says this still leaves others - around one fifth of the total - which are genuine. After initially believing they were connected with flying saucers, two years ago he came to the conclusion that they were formed by the Earth's magnetic field, which somehow electrocuted the crops, making the stalks lie down in neat circles.

Among those who doubt the non-human theories is David Sutton, managing editor of the Fortean Times, the journal of the strange and the unexplained. But even he is reluctant to dismiss the paranormal possibilities altogether.

"It is a vexed issue. There are many people who claim to make these circles, but no matter how many times they publicise their claims, there is still a great resistance among some to believe that they are made by people," he says.

"But there is an interesting dance between the two groups of people - those who describe themselves as artists and the circle researchers - and I believe they really sustain each other: it is in everybody's interests if it retains an air of mystery. "

He says the reason for the UK's pre-eminence in the field may have something to do with our national characteristics. "They seem to be a peculiarly British thing, some way between a piece of land art and a prank, and we have got a lot of white horses to provide inspiration, but whatever the reason, I believe they are ultimately enriching.

"They have given more fun to more people than most art forms, and it is fascinating to watch the New Agers believing they can be healed by sitting in them."

For the Fortean Times, crop circles have become an annual tradition, and it must be difficult not to become a little jaded. But ultimately, Sutton says, the question may be less about why they are here, than simply a matter of appreciating the fact they are.

"I remain sceptical of all paranormal claims unless people back them up with evidence. But whether you believe or don't believe, the fact that people have strong feelings is fascinating in itself.

"And who am I to say how they got here? I have never seen one being made, whether by a bunch of guys with planks or by a plasma ball. But whatever you attribute them to, they are fascinating things."

* Signs is showing in cinemas in the region now.