Other explanations

Weather

Some people have suggested that crop circles are the result of extraordinary meteorological phenomena. This hypothesis probably originated from a 1880 publication in Nature by investigator and amateur scientist John Rand Capron. Part of the publication reappeared in the January 2000 issue of Journal of Meteorology:

"The storms about this part of Western Surrey have been lately local and violent, and the effects produced in some instances curious. Visiting a neighbour's farm on Wednesday evening (21st), we found a field of standing wheat considerably knocked about, not as an entirety, but in patches forming, as viewed from a distance, circular spots... I could not trace locally any circumstances accounting for the peculiar forms of the patches in the field, nor indicating whether it was wind or rain, or both combined, which had caused them, beyond the general evidence everywhere of heavy rainfall. They were suggestive to me of some cyclonic wind action..."

Paranormal

Since appearing in the media in the 1970s, crop circles have become 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.
The location of many crop circles near ancient sites such as Stonehenge, barrows, and chalk horses has led many New Age belief systems to incorporate crop circles, speculating their existence in relation to ley lines.
Some New Age supporters have related crop circles to the Gaia hypothesis, alleging that "Gaia", the earth, is actually alive and that crop circles are messages or responses to stimuli such as global warming and human pollution. It asserts that the earth may be modeled as if a single super-organism, in that earthly components (e.g. biota, climate, temperature, sunlight, etc.) influence each other and are organized to function and develop as a whole.
The main criticism of alleged non-human creation of crop circles is that while evidence of these origins, besides eyewitness testimonies, is essentially absent, some are definitely known to be the work of human pranksters and others can be adequately explained as such. There have been cases in which researchers declared crop circles to be "the real thing", only to be confronted with the people who created the circle and documented the fraud (see above). In his 1997 book The Demon-Haunted World: Science as a Candle in the Dark, Carl Sagan discussed alien-based theories of crop circle formation. Sagan concluded that no empirical evidence existed to link UFOs with crop 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 to create a crop circle are now well documented on the internet.
Responding to local beliefs that "extraterrestrial beings" in UFOs were responsible for crop circles appearing in Indonesia, the government and the National Aeronautics and Space Agency (Lapan) described them as "man-made". Thomas Djamaluddin, research professor of astronomy and astrophysics at Lapan stated: "We have come to agree that this 'thing' cannot be scientifically proven. Scientists have put UFOs in the category of pseudoscience."

Animal activity

In 2009, BBC News reported that Lara Giddings, the attorney general for the island state of Tasmania, stated that Australian wallabies had been found creating crop circles in fields of poppies after consuming some of the opiate-laden crop and running in circles.