Despite films like Signs, there is no evidence that crop circles are made by alien intelligences. Hoaxing is by far the best explanation for crop circles—far more so than aliens who supposedly travel across the vast universe to reach Earth, only to flatten wheat in rural English and American farms as some sort of information. (You’d think that aliens with superior intelligence would realize their messages aren’t getting through, and be a little more direct.)

Further reading:

Kevin Christopher. "CSICOP Crop Circle Experiments." Available at http://www.csicop.org/hoaxwatch/cropcircles.html.