Forget crop circles. Winter's got its own mysterious phenomenon: perfect circles of ice that slowly spin in place in the water. Putting aside alien encounter jokes, there are several different theories about what causes this spectacle. The leading suggestion is that slow river currents can twist a piece of ice around; as the ice rotates, its edges are gradually worn down to form a perfect circle.

This 6-foot wide, spinning circle was discovered in Canada in 2008.