The BBC's Jane O'Brien: There are unexplained factors such as the lack of footprints

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.

The first few crop circles of the season have already appeared in several West Country fields. The area has long been the focal point of those in Britain who believe that the circles are the work of extra terrestrial forces.

Last year a US Website advertised 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.

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One of Mr Rockefeller's areas for charitable giving is what he calls "spirituality", which includes research into UFOs and other unexplained phenomena.

Work funded by the billionaire has already built up the biggest crop circle database.

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, 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.

In a BBC CountryFile special in January, Mr Bower, 74, showed how his patterns were made with planks of wood, lengths of rope and a ball of string. He said he was amazed that many followers of crop circles still refused to believe they were a hoax.

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

An earlier version of this story included a picture of Larry Rockefeller, Laurance Rockefeller's son. Larry Rockefeller has no involvement in funding the crop circle research and BBC News Online regrets any confusion.