'One couple were blown off their feet when a circle suddenly formed round them in the middle of a walk'

What price the huge crop circle in the wheat beside the A34 near Litchfield, in Hampshire? Natural formation or hoax?

The fact that it lies on a hillside in full view of a main road at once raises the hackles of a sceptic. Why should it be there, and not on some tucked-away slope where no one but the farmer would see it? On the other hand, it is a huge and complex pattern, 80 yards in diameter. It has seven concentric rings of flattened corn, each a yard wide, and then a broad, scalloped perimeter, with swirls of laid wheat swinging in and out. If humans did make it, they worked with extraordinary skill and dispatch, and produced a perfectly symmetrical design.

Since it appeared during the night of 6-7 July, it has been intensively fancied by specialists, not least the indefatigable Lucy Pringle, vice- chairman of the Centre for Crop Circle Studies, who is keen to establish by scientific tests whether or not it is natural.

"Of course, the hoaxers have been about again this year," she says. "But we can now say 'Hoaxers go home!' because science has shown us how to distinguish between what is real and what is phoney."

According to Mrs Pringle, 1995 has been a good year for crop circles: although numbers are slightly down on last season, the quality has been higher. The earliest formation, which appeared in oil-seed rape in Lancashire during April, was soon dismissed as a fake, but there have been many genuine ones in the South.

For serious researchers, the main novelty is that ADAS, the Government's farming advisory service, has started to take an interest and is calling for samples of plants and soil taken from formations. This has made things easier for investigators, since farmers more willingly allow them on their land.

Research in America has already suggested that circles may be formed by sudden surges of heat or energy, which travel up the plant stalks and break down the molecular structure inside. Often the charge merely stretches the nodes, or joints, so that the stalks bend over, but sometimes it is so powerful that it forces its way out through little expulsion cavities. "It's just like a microwave," Mrs Pringle explains. "If you put a jacket potato in, it'll cook, but if you give it too much force, it'll explode."

Odd changes have also been found in seedheads. If a charge hits them when they are young, they appear to end up with polyembryony - that is, containing nothing but multiple embryos. Seeds hit late develop normally, even if they look wizened; and these, if planted again, generate more quickly and grow into stronger plants than grain from outside the circle.

Mrs Pringle's particular interest lies in the effect that formation-creating forces may have on water - and earlier this week I went with her to recover eight small bottles which she had buried in the Litchfield circle soon after it appeared: six inside and two (as controls) outside.

Precisely recorded information enabled us to find most of them without difficulty. "North perimeter, ring two, east of the tramline" and there it was, a couple of inches down in the stony soil. Only the bottle in the centre had disappeared, dug up by some unscrupulous prowler.

As she worked, taking careful notes, Mrs Pringle enthused about the season's finest formations - the Jellyfish, the Clutch-plate, the Southern Cross. She mentioned how, in the circles, electrical equipment frequently fails: mobile telephones collapse and batteries go dead.

She described the incident in which two labradors went berserk and attacked each other and then humans, as if a pulse on a particular radio frequency had sent them temporarily round the bend. Most eerily of all she recalled how in the Eighties, a couple called Tomlinson, who had never heard of crop formations, were blown off their feet and had their eardrums perforated when a circle suddenly formed round them in the middle of a walk. All these phenomena, and many more, will feature in her forthcoming book The Sky Is Not the Limit.

As the sun beat down on the ripening wheat, I found my imagination racing - and when a helicopter suddenly made a low pass over us, I thought for a moment that alien forces had us under observation. Analysis will show whether or not Mrs P's bottles have a tale to tell (earlier tests on water from another formation have shown appreciable differences). All your correspondent can report is that he returned from the expedition with his torso breaking out into a nasty and inexplicable rash.