Take a look at the circle in the picture above. This is an ice circle, a rare natural phenomenon in which large discs of ice are formed in slow moving water in cold climates. And the interesting thing is that those ice circles rotate slowly in the water. That's why many UFO-enthusiasts claim that ice circles are actually signs left by visiting aliens!
Ice circles normally occur at bends in the river where the accelerating water creates a force called 'rotational shear', which breaks off a chunk of ice and twists it around. As the disc rotates, it grinds against surrounding ice and the result is a perfect circle that people call "Ice Circle". Another kind of ice circle are actually ice pans, or surface slabs of ice that form in the center of a lake or creek, instead of along the water's edge. They can be explained by quick shifts in temperature. As water cools, it releases heat that turns the water into frazil ice — a collection of loose, needle-shaped ice particles that can cluster together in an ice pan. Wikipedia
Ice circles have most frequently been observed in Scandinavia and North America, but was also recorded in Britain and Russia. Want to know more about ice circles? Take a look at the following photographs. Also in the end there is a video. Watch it!
River Otten in Devon, UK (Source)
Sheridan Creek, Rattray Marsh in Ontario, Canada (Source)
Hemlock River located near Amasa, Michigan (Source)
Piteälven south of Älvsbyn in northern Sweden (Source)
Lake Baikal in Russia (Spotted by ISS astronauts)
Lake Sannavagen at Sanna in Halsingland, Sweden (Source)
Norwalk, Connecticut, USA (Source)
Wescott Stream, Waldo, USA (Source)
Råneälven in Norrbotten, Sweden (Source)
Some more pictures of ice circles spotted in different Russian locations.(Source)
Also take a look at the ice circle video. It shows the ice circle that appeared in Rattray Marsh, Mississauga, Ontario, on December 13th, 2008.