Hoax Theory
As interest in crop circles peaked in the early 90s, two retired Englishman named Doug Bower and Dave Chorley announced to the world that they had created all of the crop circles observed since 1978 as a joke. The two claimed that they had been creating the crop circles in the dark of the night to fool people into believing that aliens were coming. They explained their methods of making these intense designs.
By pounding a stake into the ground and tying a string to it, the two could walk around and mark a perfect circumference for their circles. Using long boards or pipes, they could flatten all of the grass in the circle. They also displayed a modified baseball cap with blinders on the side which they used to walk perfectly straight lines for some designs. They even explained that they pole vaulted into fields to disguise any signs of entering the field. Since their reitrement from hoaxing, others have stepped up to claim creative responsibilities for crop circles.
Although these man-made designs are truly impressive in their organization and execution, there are a few features which researchers like Colin Andrews use to distinguish these fakes from the real deal. Usually, careful observation will reveal that even the most careful crop fakers will leave footprints or a path of some sort, whereas no amount of scrutiny will reveal signs of entry at a real circle. The other distinguishing factor is the way that the crops are bent over. Crops bent over with a board do not share the same characteristics seen in authentic circles. This generally disproves humans as being creators of al crop circles, but it does not necessarily prove that extraterestials are the designs. There is one other theory, the Gaia theory, which claims that an intelligent being is the cause of crop circles.