A rare phenomenon usually only seen in extremely cold countries, scientists generally accept that Ice Circles are formed when surface ice gathers in the center of a body of water rather than the edges. A slow moving river current can create a slow turning eddy, which rotates, forming an ice disc. Very slowly the edges are ground down until a gap is formed between the eddy and the surrounding ice. These ice circles have been seen with diameters of over 500 feet and can also at times be found in clusters and groups at different sizes. (Photo by Brook Tyler) (Link)