By 100+ pranksters. Do you likewise love to know how spray-painted graffiti get made in the millions all over the world?
It's interesting to note that the number, location, and timing of crop-circle appearances follow the same pattern as the spread of any other fad. For example, local outbreaks of crop circles follow local media coverage of prior crop circles.
That doesn't account for the ones that are bent, not broken, and almost woven together. Some crops have undergone chemical changes, or show evidence of being under extreme energy burst. I've seen all the plank and string guys, but a lot of the circles still show signs of having been exposed to something altogether different.