There are many theories, but after some long naps I think there is only one which has enough serious evidence to support it.
> Aliens/UFOs - not enough proof that they exist
> Time Travellers - no proof that it is possible
> Hoaxers - not the big, complex ones.
> Nature - not the big, complex ones.
The answer is the Air Force. I'm not sure whether the first circles were the Air Force, and the hoaxers then got going, or the other way around. But it's fairly clear that today's complex designs are made by a computer/machine.
Why the Air Force? Probably a combination of needing real world tests of their microwave weapons, and the government's desire to distract us with weird mysteries.
Although I thought it up myself, others have had the same idea and have already researched it. See this
chapter of
UFOs - The Military Unmasked by Emmanuel Dehliner, and
this page at
Orwell Today.
Those pages contain evidence like this:
Research by Dr Levengood (of BLT Research Team), an American biophysicist, corroborated by the analyses of Ken Larsen, a British biologist, has shown that the way in which the stalks (wheat, rapeseed…) are flattened without being broken or damaged is typical of a UHF microwave effect.
Finally, some
evidence of why it must be humans, and not the hoaxing kind:
One of the most celebrated cases involved the British Prime Minister's country residence. This location, consisting of several dozen acres, is guarded against terrorist attack by Britain's most expert crack troops. It is a maximum security zone patrolled 24-hours a day. Yet despite these precautions, a Celtic cross design appeared in a field in from of John Major's house, one of the circles transformed into an arrow pointing directly at the residence. The media were quick to ask the Prime Minister if a serious breach of security had taken place. "No," he replied, "it was all merely the result of poor soil conditions."