They could be caused by any of the following:-
- Humans using boards on their feet to make patterns in crops
- Natural Phenomenon - possibly made by balls of lightning or other weather phenomenon
- Extra-terrestrial life forms or balls of light
- Created by lasers from some space based system
It seems that the more complex crop circles are probably formed by humans, but the initial circles, which were simple and round had may unexplained and unnatural phenomenon associated with it.
