Some conspiracy theoriest speculate that crop circles are being made by satellite transmitters that have been programmed with mind control messages.  That seems rather far out until you read this:

The 2009 crop circle ‘season’ started with
abundance and now looks set to bring a summer of circles to Britain’s
fields.

Whether made by human hands or an altogether
different life form, no less than 20 formations have been spotted since the season began in April. This week alone two
huge designs have mysteriously surfaced.

An intricate 150ft dragonfly appeared in a
barley field near Yatesbury, Wiltshire, just days after a jellyfish design was
cut into crops in Oxfordshire.

Alien intervention? The enormous dragonfly formation appeared in Wiltshire this weekWhen the 600ft Portuguese man-of-war turned up
near Kingstone Coombes, Oxfordshire, crop circle author Karen Alexander said:
‘This is the first jellyfish crop circle in the world. It is absolutely huge –
roughly three times the size of most crop patterns.’
Last week a 350 ft yin-yang symbol was cut into
a barley field near Devizes, a crop circle hot spot.

more here… and also see Crop Circle Connector which has lots of photos.

With over 20 crop circles in the past two months, the crop circle phenomenon is more than just a curiosity.  There is some speculation that the circles are being produced by the millitary using microwave technology.

The crop circles appear in areas that are near ley-lines, where the electromagnetic energy of the earth is said to be more powerful and there is evidence of higher EMF levels inside the circles.  This is leading to suspicion among some esoteric researchers that whoever is creating the circles is attempting to disrupt the natural force of the ley lines at these sites.  
More from Beliefnet and our partners