Adrian Potts, landlord of the Barge Inn outside Pewsey in Wiltshire, was pretty cheerful yesterday morning. The pub, on the Avon Kennet canal, is the centre of the British crop circle industry and each year, starting in early May and running for about four months, circle fans (and circle-makers) turn up from round the world. The backroom of the pub has circles on the ceiling, news of the latest formations is posted, and the chat revolves around the images that have been appearing for more than 20 years in the fields of southern England.

Potts has sold many pints on the back of crop circles, but for the last few years the Barge has been quiet. "In the late 1990s and through to 2002, [the crop circle trade] was massive. On a sunny day we'd have people here by 10am. But it dropped off and the last few years have been dreadful. The last two summers have been terrible - both in terms of weather and of circles," he says.

There are many explanations as to why the circles barely appeared in Wiltshire. The suicide of one of the chief circle-makers in 2006 and the death of two others, as well as boredom in the ranks of pranksters, have all been cited. Mostly, though, it is thought that heavy rain and high winds have made crops hard to handle and have deterred aliens and humans alike.

But just as the demise of the peculiarly English rural tradition was predicted, the circles - which can take the shape of DNA structures, scorpions, snowflakes, helices, webs, knots and complex geometric patterns - have abruptly returned in force.

The 2009 season began in April with an unprecedented six formations. The first was a series of simple circles in a field of rape; then came a 350ft yin-yang symbol in a barley field near Devizes. Three ambitious formations were reported over the last bank holiday and on Tuesday this week a giant 600ft jellyfish was found in a barley field on Bill and Sally Ann Spence's farm near Kingston Coombes in Oxfordshire.

As of yesterday, there have been more than 20 major formations spotted. Potts, who could claim to be something of an expert on the subject, has a hunch that this will be a good summer for circles: "The crops are not true enough yet. Weather permitting, I'd say the best ones will start now. In the next two weeks there should be a burst of activity."

Francine Blake, who founded the Wiltshire crop circle study group in 1995, shares Potts's optimism. She and other self-appointed investigators identify, measure, photograph, and report on all formations. They go circle-spotting at night in likely places, send crop stalks for chemical analysis in university laboratories (yes, really), and have more than 6,000 crop circles on their database.

She was excited by the jellyfish: "It's fantastic. When we look at it, it's got seven small circles, or moon shapes. It's describing the magnetic field of Earth," she says. She too is optimistic about the summer ahead: "This year started much earlier. There's one every day now. It is very intense already. I have never seen such complex designs in rape in all my years of studying this subject. Usually, the season starts with a nice little pattern, a tri-petal flower or such like, one or maybe two in rape if we are lucky. But this year they are big, complex and numerous right from the start."

"What does this mean?" Blake asks on her blog. "It means that we have to take note that something extraordinary is happening. Crop circles are not normal occurrences, they do not fit in too well with our usual beliefs. This of course is not to everyone's liking - it is not easy to face the unknown."

From this you may gather that Blake eschews more prosaic explanations for the circles. In fact the professional circle world divides neatly. One hemisphere is occupied by questors, spiritualists and paranormalists, such as Blake; the other by makers, tricksters and artists.

The first group can tell from post-holes, foot tracks, and other clear signs that the majority are made by humans. But they argue that many crop circles - perhaps 20% - defy rational explanation. Their research suggests that "true circles" are created in a very few minutes by a blast of energy. According to some, the crop cells become swollen and are bent down at the nodes, or joints. Others say the cellular structure of the plants is affected and that the composition of the soil is altered. A few circles, they say, display a phenomenal level of complexity and would be difficult to draw on paper, let alone in a field after dark.

When they cannot explain what they see, they turn to UFOs, aliens, symbols, alchemy, ancient wisdom, sacred geometry, whirlwinds, the fingerprints of God or unknown "entities" to explain what they say are messages from extra-terrestrials or signs and portents of the times.

The second group is made up of artists and pranksters. What began in the late 1970s with two Wiltshire watercolour artists, Doug Bower and Dave Chorley, going into the fields for a laugh to create simple circles to tease those who believe in UFOs, was picked up by London-based artists and sculptors in the mid-1990s.

One group, now calling themselves the Circlemakers, includes situationist artists Rod Dickinson and John Lundberg, the sculptor Gavin Turk, Rob Irving and others. They say on their website that they latched on to the circle believers, created images from reading the same books that the believers read, and that they now team up with other teams of circle-makers to create ambitious formations. Together they say they have made crop circles an essential part of our popular culture, part of the myth of the English countryside.

"We weren't pushing paint around on a canvas that sat in a sterile gallery environment; we were quite literally forming and shaping the culture that surrounded us," Lundberg said in 2004. "We are the heretics, calling their belief system into question by the mere fact that we exist and talk about our circle-making activities. Sometimes this spills over into threatening behaviour on the part of the believer. We've had potatoes stuck up our exhausts, wing mirrors ripped off of our cars, and threats of physical violence, in person, over the phone, via email and through our letterboxes."

For a long time the Circlemakers kept their identities secret but they now openly claim to have made many hundreds of circles. However, they play the game that there is some inexplicable force out there by not claiming to have made them all, and never revealing which particular ones they created.

Those in the other camp are adamant the Circlemakers are destructive. "They used to call themselves Team Satan. They live in south London and it takes them days to make them even in daylight. They have nothing to do with the phenomenon," says Blake.

The other reason why there may have been fewer circles in recent years is that leading circle-makers are growing up, and can now command big money. Formations are now regularly commissioned by multinational companies, advertising agencies and the media. Nike, Pepsi, BBC1, Greenpeace, Sky, Weetabix, Big Brother, Mitsubishi, Thompson Holidays and O2 have all paid circle-makers tens of thousands of pounds for a night's work. They have been made for pop videos, corporate parties, TV dramas and ads. The Sun paid for one to publicise its campaign to bring the Olympics to Britain.

From being genuinely intriguing, amusing and innocent folk art, the formations have become worth millions of pounds to the Wiltshire tourist industry. Farmers, too, can make thousands of pounds, either in compensation from companies wanting to have their logos plastered in their fields, or from charging people £2 each to walk in a circled field. One farmer near Stonehenge is said to have made about £30,000 by charging tourists to visit circles on his land.

The "believers" also make money from conferences, books, magazines, and calendars, lectures and sightseeing tours. "A good aerial picture of a sophisticated circle picked up by TV or the press can make tens of thousands of pounds," says the head of one picture agency, who asked not to be named. "Everyone is at it."

Of course some farmers are furious to find their crops flattened. They do tend to stand up again, however, after they've been bent over; normally this is enough to smooth most feathers. And no one is suggesting that this year's circles have been commissioned by tourist boards, or have been sponsored by corporations. But it's more than likely that someone will make money from the photographs, the field, or the design of these latest additions to the oeuvre.

Blake dismisses any idea that the phenomenon is driven by art or by money. "Something important is happening. It's raining shapes every day now. Nothing man-made could be like this. That's why people can't get their heads round it."


How to make a crop circle

• Prepare a detailed drawing. Keep it simple. Circles and triangles are relatively easy to make. Advanced curves, spirals, straight lines, fractals and pictures can take a long time to mark out and work.

• You will need helpers; decide who will do what and in what order the image needs to be constructed.

• You will need a marked rope or a 100ft measuring tape to mark out the site, and a foot-wide wooden board about 4ft long to do the flattening. The board should have ropes attached to each end so you can loop it over your neck.

• Ask permission from a farmer and be prepared to pay compensation. A crop circle can cause hundreds of pounds of damage.

• Wait for a moonlit night when it is dry. Enter the field by the tramlines, or marks left by tractors.

• Mark out the field carefully. Some circle makers use sticks or poles but these can leave tell-tale holes of human intervention.

• Put the rope round your neck, with the board on the ground in front of you; press down with your right foot, move it forward, press it down again, and so on.

• Leave the field the same way you entered.

• Hide your tracks.