One popular theory is that they are created by extraterrestrials who are trying to give us signs, warnings, or other information. The extraterrestrial explanation is of course very popular, as Lee Speigel of AOL News noted: "One resident, Cahyo Utomo, speculated that the odd piece of rice field artwork wasn't man-made. ‘The circles were there since yesterday morning. I think they were left by an alien spaceship,' he said."
If this explanation is true, it seems very strange that aliens would go to the trouble of traveling across the universe to our pale blue dot just to press down some plants in various shapes and then leave. Surely if an alien civilization is advanced enough to master space travel, it can devise more effective ways of communicating with us.
There's only one known source for crop circles: humans. (In fact, I have personally investigated and made several crop circles, as I discuss in my 2010 book "Scientific Paranormal Investigation: How to Solve Unexplained Mysteries.") Many people believe that crop circles have been reported for centuries, mistakenly citing a folktale from 1678 in which an English farmer told a worker with whom he was feuding that he "would rather pay the Devil himself" to cut his oat field.