Human pranksters are responsible for most crop circles in the world, but as with UFO phenomena, a certain number defy any logical explanation. One thing for certain, they're becoming more frequent, with at least one made every day somewhere in the world.
According to LiveScience.com:
Crop-circle enthusiasts have come up with many theories about what creates the patterns, ranging from the plausible to the patently absurd. One explanation in vogue in the early 1980s was that the circles were accidentally produced by the especially vigorous sexual activity of mating hedgehogs. As the patterns became more complex that idea was abandoned, but some of the theories that replaced it were equally outlandish. Some people have suggested that the circles are somehow created by incredibly localized and precise wind patterns, or by scientifically undetectable Earth energy fields and meridians called ley lines.
. . . While there are countless theories, the only known, proven causes of crop circles are humans. Many people believe that crop circles have been reported for centuries (for example mistakenly suggesting that a 1678 woodcut of a folkloric legend about Satan harvesting wheat is evidence of a crop circle); but in fact the first ones appeared only in the 1970s. Their origin remained a mystery until September 1991, when two men confessed that they had created the patterns for decades as a prank to make people think UFOs had landed. They never claimed to have made all the circles — many were copycat pranks done by others — but their hoax launched the crop-circle phenomenon. Most crop-circle researchers admit that hoaxers craft the vast majority of crop circles. But, they claim, there's a remaining tiny percentage that they can't explain.
The LiveScience.com article explores the attributes common to most crop circles and what are some of the suspected causes.