Crop circles have become a feature of the UK countryside in Summer, especially in Wiltshire and Hampshire.

But what causes them? Are they all fakes? Are they caused by magnetic shifts? Or are they UFO landing sites?

Every summer you can be sure of two things in the UK. Rain and crop circles. So prolific are they that a few years ago US billionaire Laurance Rockefeller funded a huge UK survey of crop circles.

Scientists carried out aerial research over Wiltshire in the hope of finding out once and for all whether the mysterious patterns were genuine or the work of hoaxers.

Their research has added greatly to the study of crop circles, but has come up with no firm conclusions.

Some believe they are created by UFOs during nocturnal visits. Others say they are connected to ancient "ley lines", or put it down to natural phenomena such as unusual forms of lightning.

US websites have been known to advertise week-long tours of UK crop circles priced at $2,199 per person.

Until now research has been carried out by amateurs and enthusiasts, known as croppies. But there is a growing scientific discipline based around the study, known as cereology.

Less concerned with how they're made, Lucy Pringle's exhaustive work concentrates on the effects crop circles have on those who come close to them.

One such expert is Lucy Pringle, who was a founder member of the Centre for Crop Circle Studies.

Her research revolves around effects on people, both psychologically and physically, who have come into contact with a crop circle.

She also measures possible changes in animal behaviour where crop circles appear.

Many farmers believe crop circles are the work of hoaxers, and say they cause thousands of pounds of damage every year. Several people have come forward to claim responsibility.

In 1991 two landscape painters from Hampshire, David Chorley and Douglas Bower, claimed they started the hoax in 1978, after drinking in a pub.

They said for the past 13 years they had been sneaking around southern England at night, fashioning as many as 25 to 30 new circles each growing season.

But it seems there remain unexplained factors, such as the lack or tracks or footsteps.

Colin Andrews' believes 80% of crop circles are man made, and others are formed by magnetic shifts.

Colin Andrews has researched crop circles since the 80s.

His research claims the appearance of crop circles in summer fields are caused by the Earth's magnetic field.

Colin says 17 years of work has revealed that about 80% of the formations are man-made.

But he believes that magnetism may account for the rest, which display a simplicity of form compared with elaborate, beautiful patterns of the "hoaxes".

Dr Andrews believes a mysterious shift in the magnetic field gives rise to a current that "electrocutes" the crops forcing them to lie flat on the ground.

John Lundberg makes crop circles....but has he been told to by aliens?

Self-proclaimed hoaxer John Lundberg said no-one would ever believe a scientific explanation for crop circles because people want to believe it is something more mysterious. "The public don't want it explained," he said.

Some argue that even so-called 'fakers' have been instructed to create patterns subconsciously by aliens wanting to send earth messages in a process called 'auto-rolling'.

Most mainstream scientists believe the only explanation for crop circles lies in the footboards of hoaxers.