Nice site with pretty pictures and a history of when and where the various patterns were formed.
http://www.lovely.clara.net/history.html
Forget the cranks - compare the self confessed man made circles found at
http://www.circlemakers.org/ with the "unexplained" examples above.
Which ones hold your attention for longer?
Do all look man-made.
I know that when I'm aware a crop circle is man-made, I notice more flaws, less finesse to the curves and linear detail.
God only knows!
Doug
