Many people think crop circles are made by humans. But others argue that some have appeared so quickly, it would be impossible for humans to make them.
Experts have examined the bent grain stalks and the soil inside the circles, and often claim to find anomalies. For example, some say they have found more magnetic iron compounds in the soil inside the circles than outside. However, these results are only meaningful if the examinations are carried out as part of a controlled experiment.
The two principal alternatives to the explanation that crop circles are made by humans are that they are made by extraterrestrials, or that they are made naturally by wind vorticies.