I've always been intrigued by Crop Circles.
I actually saw this one in person: http://i2.squidoocdn.com/resize/squidoo ... _henge.jpg
Supposedly a "real" crop circle as opposed to an obvious hoax has perfect circles, made of plants that are bent but not broken and are still alive. There is also unusual activity with the water table below the circle.
I find it hard to believe that some of the more complex ones are made by some purely random natural force, yet am at a loss to explain who or what could actually make them, and just as importantly, why?