I don't know. I suspect that it is some form of life that is plasma based. We perceive them as globes of light. They are trying to tell us something, but we just don't seem to get it, yet.
I was always impressed with Jacques Vallee’s thinking on the UFO business; and he has written about this, too. What he says seems important in ways that go beyond ‘crop circles’:
http://boingboing.net/2010/03/23/in-search-of-alien-g.html#previouspost
http://boingboing.net/2010/04/08/crop-circles-part-de.html#previouspost
http://boingboing.net/2010/04/28/of-flattened-flora-a.html
http://boingboing.net/2010/06/21/of-crop-circles-meme.html