The puzzling formations known as crop circles have appeared worldwide throughout history. In the Middle Ages, they were called "witch" or "pixie" circles, and a 1678 woodcut, the "Mowing Devil", depicts one thought to be Satan's work. But in the 1980s, the phenomenon escalated, with dozens of crop circles popping up in England and other countries. When two Englishmen claimed they had perpetrated the hoax, many felt the riddle was solved. And yet, more have materialized. We explore the mystery.