Super information from http://www.viewzone.com .

We've all heard about crop circles that appear in the middle of corn or wheat fields. But here is a little known phenomenon -- ice circles! These perfectly circular formations have been recorded for some time now, usually appearing on rivers or small ponds, but occasionally also in the calm ocean waters on enormous scales.

This circle appeared overnight in a small irrigation ditch in Delta, Ontario. Notice the appearance of a groove outlining the formation.

The above two photos were taken of the same circular formation as it drifted down a river in Amasa, Michigan in 2006. Notice it is surrounded by liquid water. The edges look as if they were also carved but the origin of this formation is still a mystery.

The above photo is from Norwalk, Connecticut in 2003. The formation was seen in a park. The surrounding ice was too thin for anyone to have walked on it and made the circle.

The formation (above top) is typical of smaller circles often seen on streams. In the detail photo (bottom) of the same circle we see how the edges appear to have been cut, yet no other signs or tracks nearby shed light on how this was done.

These photos show a circle located in Northeast Utah. Again, it appears to have been cut but the fragile ice surrounding it shows no signs of human tracks.