The Skeptic's Dictionary
crop "circle" is a geometric pattern, often very intricate and complex, appearing in fields, usually wheat fields and usually in England. Most, if not all, crop circles are probably due to pranksters. For example, Doug Bower and David Chorley admit to hoaxing approximately 250 circles over many years.
Some believe that the crop designs are messages from alien spacecraft. Some maintain that the aliens are trying to communicate with us using ancient Sumerian symbols or symbolic representations of alien DNA. Those who engage in such serious study and theorizing about crop circles are known as cerealogists (after Ceres, the Roman goddess of agriculture and fertility) or croppies.
Even scientifically minded people have been brought into this fray. They have wisely avoided the thesis that aliens have been carving out messages in crop fields. But they have stretched their imaginations to come up with theories of vortexes, ball lightning, plasma, and other less occult explanations involving natural forces such as wind, heat, or animals. Some think the designs are clearly the work of the U.S. Air Force and the RAF using a "military microwave cannon, piloted by computer," and a design book.* However, when looking for an explanation of weird things we should never omit from our checklist the possibility that the phenomenon we are studying is a hoax.
Had crop circles existed in the thirteenth century, they would have been attributed to Satan, who was said to have been responsible for many weird happenings as well as for many unweird things, such as the construction of Stonehenge and Hadrian's wall between England and Scotland. It was believed by many that the ancients could not possibly have accomplished such feats on their own. Today, Satan's power as an explanation for weird or wondrous things has been usurped by aliens.