thumb|upright=1.2|Aerial view of crop circles in Switzerland

A crop circle, crop formation, or corn circle is a pattern created by flattening a crop, usually a cereal. The term was coined in the early 1980s. Crop circles have been described as all falling "within the range of the sort of thing done in hoaxes" by Taner Edis, professor of physics at Truman State University.

Although obscure natural causes or alien origins of crop circles are suggested by fringe theorists, there is no scientific evidence for such explanations, and all crop circles are consistent with human causation. In 1991, two hoaxers, Doug Bower and Dave Chorley, took credit for having created over 200 crop circles throughout England in widely-reported interviews. They usually appear overnight. Nearly half of all crop circles found in the UK in 2003 were located within a radius of the Avebury stone circles. In 1991, meteorologist Terence Meaden linked this report with modern crop circles, a claim that has been compared with those made by Erich von Däniken.

An 1880 letter to the editor of Nature by amateur scientist John Rand Capron describes how several circles of flattened crops in a field were formed under suspicious circumstances and possibly caused by "cyclonic wind action", stating "as viewed from a distance, circular spots ... they all presented much the same character, viz, a few standing stalks as a centre, some prostrate stalks with their heads arranged pretty evenly in a direction forming a circle round the centre, and outside there a circular wall of stalks which had not suffered".

20th century

In 1932, archaeologist E. C. Curwen observed four dark rings in a field at Stoughton Down near Chichester, but could examine only one: "a circle in which the barley was 'lodged' or beaten down, while the interior area was very slightly mounded up."

In Fortean Times, David Wood reported that in 1940 he made crop circles near Gloucestershire using ropes.

In 1963, Patrick Moore described a crater in a potato field in Wiltshire that he considered was probably caused by an unknown meteoric body. In nearby wheat fields, there were several circular and elliptical areas where the wheat had been flattened. There was evidence of "spiral flattening". He thought they could be caused by air currents from the impact, since they led towards the crater. Astronomer Hugh Ernest Butler observed similar craters and said they were likely caused by lightning strikes.

During the 1960s, there were many reports of UFO sightings and circular formations in swamp reeds and sugarcane fields in Tully, Queensland, Australia, and in Canada. The most famous case is the 1966 Tully "saucer nest", when a farmer said he witnessed a saucer-shaped craft rise from a swamp and then fly away. On investigating he found a nearly circular area long by wide where the grass was flattened in clockwise curves to water level within the circle, and the reeds had been uprooted from the mud. The local police officer, the Royal Australian Air Force, and the University of Queensland concluded that it was most probably caused by natural causes, like a down draught, a willy-willy (dust devil), or a waterspout. In 1973, G.J. Odgers, Director of Public Relations, Department of Defence (Air Office), wrote to a journalist that the "saucer" was probably debris lifted by a willy-willy.

After the 1960s, there was a surge of UFOlogists in Wiltshire, and there were rumours of "saucer nests" appearing in the area, but they were never photographed.

The first film to depict a geometric crop circle, in this case created by super-intelligent ants, was the 1974 science-fiction film Phase IV. The film has been cited as a possible inspiration or influence on the pranksters who started this phenomenon.

The majority of reports of crop circles have appeared and spread since the late 1970s as many circles began appearing throughout the English countryside. Around this time, researcher Colin Andrews began documenting the phenomenon, and in 1989 he co-authored Circular Evidence with Pat Delgado,

Although farmers expressed concern at the damage caused to their crops, local response to the appearance of crop circles was often enthusiastic, with locals taking advantage of the increase of tourism and visits from scientists, crop circle researchers, and individuals seeking spiritual experiences. The market for crop circle interest consequently generated bus or helicopter tours of circle sites, walking tours, T-shirts, and book sales.

21st century

Since the start of the 21st century, crop formations have increased in size and complexity, with some featuring as many as 2,000 different shapes

The researcher Jeremy Northcote found that crop circles in the UK in 2002 were not spread randomly across the landscape. They tended to appear near roads, areas of medium-to-dense population, and cultural heritage monuments such as Stonehenge or Avebury. He found that they always appeared in areas that were easy to access. This suggests strongly that these crop circles were more likely to be caused by intentional human action than by paranormal activity. Another strong indication of that theory was that inhabitants of the zone with the most circles had a historical tendency for making large-scale formations, including stone circles such as Stonehenge, earthen mounds such as Silbury Hill, long barrows such as West Kennet Long Barrow, and white horses in chalk hills. To prove their case they made a circle in front of journalists; a "cereologist" (advocate of paranormal explanations of crop circles), Pat Delgado, examined the circle and declared it authentic before it was revealed that it was a hoax.

Inspired by Australian crop circle accounts from 1966, Bower and Chorley claimed to be responsible for all circles made prior to 1987, and for more than 200 crop circles in 1978–1991 (with 1,000 other circles not being made by them).

Since the early 1990s, the UK arts collective Circlemakers, founded by Rod Dickinson and John Lundberg, and subsequently including Wil Russell and Rob Irving, has been creating crop circles in the UK and around the world as part of its art practice and also for commercial clients.

The Led Zeppelin Boxed Set that was released on 7 September 1990, along with the remasters of the first boxed set, as well as the second boxed set, all feature an image of a crop circle that appeared in East Field in Alton Barnes, Wiltshire.

thumb|right|Aerial view of a crop circle in [[Diessenhofen]]

On the night of 11–12 July 1992, a crop-circle-making competition with a prize of £3,000 (funded in part by the Arthur Koestler Foundation) was held in Berkshire. The winning entry was produced by three Westland Helicopters engineers, using rope, PVC pipe, a plank, string, a telescopic device and two stepladders. According to Rupert Sheldrake, the competition was organised by him and John Michell and "co-sponsored by The Guardian and The Cerealogist". The prize money came from PM, a German magazine. Sheldrake wrote that "The experiment was conclusive. Humans could indeed make all the features of state-of-the-art crop formations at that time. Eleven of the twelve teams made more or less impressive formations that followed the set design."

In 2002, Discovery Channel commissioned five aeronautics and astronautics graduate students from MIT to create crop circles of their own, aiming to duplicate some of the features claimed to distinguish "real" crop circles from the known fakes such as those created by Bower and Chorley. The creation of the circle was recorded and used in the Discovery Channel documentary Crop Circles: Mysteries in the Fields.

In 2009, The Guardian reported that crop circle activity had been waning around Wiltshire, in part because makers preferred creating promotional crop circles for companies that paid well for their efforts.

A crop circle depicting the emblem of the Star Wars Rebel Alliance was created in California in December 2017 by a father and his 11-year-old son as a spaceport for X-wing fighters.

In 1992, Gábor Takács and Róbert Dallos, both then aged 17, were the first people to face legal action after creating a crop circle. Takács and Dallos, of the St. Stephen Agricultural Technicum, a high school in Hungary specializing in agriculture, created a diameter crop circle in a wheat field near Székesfehérvár, southwest of Budapest, on 8 June 1992. In September, the pair appeared on Hungarian TV and exposed the circle as a hoax, showing photos of the field before and after the circle was made. As a result, Aranykalász Co., the owners of the land, sued the teens for 630,000 Ft (~$3,000 USD) in damages. The presiding judge ruled that the students were only responsible for the damage caused in the circle itself, In November 2000, he was fined £100 plus £40 in costs. , no one else has been successfully prosecuted in the UK for criminal damage caused by creating crop circles.

Creation

thumb|Detail of a crop circle in a field in Switzerland

Human origin

The scientific consensus on crop circles is that they are constructed by human beings as hoaxes, advertising, or art. The most widely known method for a person or group to construct a crop formation is to tie one end of a rope to an anchor point and the other end to a board which is used to crush the plants. It is also possible to bend grass without breaking it, if it has recently rained—a method that was used to create crop circles in Hungary in 1992. such as Bower and Chorley and tabloid Today hoaxing Pat Delgado, or a friend of a Canadian farmer hoaxing a field researcher of the Canadian Crop Circle Research Network. In his 1995 book The Demon-Haunted World: Science as a Candle in the Dark, Carl Sagan concludes that crop circles were created by Bower and Chorley and their copycats, and speculates that UFOlogists willingly ignore the evidence for hoaxing so they can keep believing in an extraterrestrial origin of the circles. Many others have demonstrated how complex crop circles can be created. Scientific American published an article by Matt Ridley, who started making crop circles in northern England in 1991. He wrote about how easy it is to develop techniques using simple tools that can easily fool later observers. He reported on "expert" sources such as The Wall Street Journal who had been easily fooled, and mused about why people want to believe supernatural explanations for phenomena that are not yet explained. Methods of creating a crop circle are now well documented on the Internet.

Some crop formations are paid for by companies who use them as advertising. By the end of 1991 Meaden conceded that those circles that had complex designs were made by hoaxers.

Animal activity

In 2009, the attorney general for the island state of Tasmania stated that Australian wallabies had been found creating crop circles in fields of opium poppies, which are grown legally for medicinal use, after consuming some of the opiate-laden poppies and running in circles.

Alternative explanations

In science magazines from the 1980s and 1990s, for example Science Illustrated, one could read reports suggesting that the plants were bent by something that could be microwave radiation, rather than broken by physical impact. The magazines also contained serious reports of the absence of human influence and measurement of unusual radiation. Today, this is considered to be pseudoscience, while at the time it was subject of serious research. At that time, it was also more likely that an unknown factor was behind the incidents, not least seen in light of the fact that GPS was not available to the public.

Paranormal

thumb|Sketch of a "spaceship" creating crop circles, sent to UK [[Ministry of Defence (United Kingdom)|Ministry of Defence circa 1998]]

Since becoming the focus of widespread media attention in the 1980s, crop circles have been the subject of speculation by various paranormal, ufological, and anomalistic investigators, ranging from proposals that they were created by bizarre meteorological phenomena to messages from extraterrestrial beings. There has also been speculation that crop circles have a relation to ley lines.

Some paranormal advocates think that crop circles are caused by ball lightning and that the patterns are so complex that they have to be controlled by some entity. Some proposed entities are Gaia asking to stop global warming and human pollution, God, supernatural beings (for example Indian devas), the collective minds of humanity through a proposed "quantum field", and extraterrestrial beings.

Responding to local beliefs that "extraterrestrial beings" in UFOs were responsible for crop circles appearing, the Indonesian National Institute of Aeronautics and Space (LAPAN) described crop circles as "man-made". , research professor of astronomy and astrophysics at LAPAN stated, "We have come to agree that this 'thing' cannot be scientifically proven." Among others, paranormal enthusiasts, ufologists, and anomalistic investigators have offered hypothetical explanations that have been criticised as pseudoscientific by sceptical groups and scientists, including the Committee for Skeptical Inquiry. No credible evidence of extraterrestrial origin has been presented.

Changes to crops

A small number of scientists (physicist Eltjo Haselhoff, the late<!--described as semi-retired by csicop source--> biophysicist William Levengood) have claimed to observe differences between the crops inside the circles and outside them, citing this as evidence they were not man made. In his 1994 paper he found that certain deformities in the grain inside the circles were correlated to the position of the grain inside the circle.

Advocates of non-human causes discount on-site evidence of human involvement as attempts to discredit the phenomena. When Ridley wrote negative articles in newspapers, he was accused of spreading "government disinformation" and of working for the UK military intelligence service MI5.

Patterns similar to crop circles can also be made in snow, by using skis, snow shoes or just walking with ordinary shoes.

Images can be made in forests by cutting trees, especially in areas with snow. Celebrating the Olympic Games in Lillehammer, Norway in 1994, a tall stylised image of an Olympic torch runner was made in a forest close to one of the arenas.

Folklore

thumb|upright=1.1|1678 pamphlet on the "[[Mowing-Devil"]]

Researchers of crop circles have linked modern crop circles to old folkloric tales to support the claim that they are not artificially produced.