September 15, 2009--People investigate a crop circle of unknown origins in a cornfield in Corcelles-pres-de-Payerne, Switzerland, on July 7, 2007.

Crop circles, which have been described as everything from alien art forms to sly ad placements, appeared just as mysteriously on Google's home page overnight.

Crop circles first drew attention in the mid-1970s, when they were photographed from the air in fields in the county of Wiltshire, the U.K.. Made mainly from flattened cereal crops such as wheat and barley, these earliest examples were fairly simple in design.

Since then the region has played host to an increasingly elaborate portfolio of crop circles. In 2008, for instance, a 150-foot-wide (46-meter-wide) coded representation of the first ten digits of the mathematical constant Pi appeared.

Most of these gigantic works of art are certainly produced by human crop circle makers, by pulling planks attached to ropes, for example. But don't assume they all are, U.K. crop circle researcher Karen Alexander said.

"There's no doubt that some crop circles are made by people and that some are made for advertising campaigns and so forth," Alexander said. "But then there's quite a large percentage for which origin is just unknown."

--With reporting by James Owen

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