Electro-Magnetic Fields
Andrew Colins is responsible for developing and researching the Electro-Magnetic theory. He began by taking magnemometer reading of the area around a crop circle, and found "increases in the earth's magnetic field by approximately 120 percent." Furthermore, he found that when printed out, the readings showed the same patterns and designs as the crop circle present. Colins predicts that the source of the magnetic interference must be at either 20 ft below or 20 ft above the ground, but has never been able to pinpoint what the exact source is.
There are many other pieces of evidence to support this theory, such as the consistent reports of electro-magnetic interference at crop circles, and the fact that a crop's tend to thrive in areas of high magnetic activity. There are also arguments against this theory, especially the argument that magnetism occurs everywhere on earth, so if magnets caused crops to flatten, crops across the world would flatten consistently. Even critics of the Electro-magnetic theory do not deny the logic behind it, and certainly agree that it is a more likely theory than the whirlwind theory.