Earlier this year we wrote about a strange phenomenon causing circles of ice to break free and start rotating on their own. Those circles were obviously man-made, and clearly the work of someone looking for something more entertaining than boring crop circles.
Last April, NASA found something far more interesting, and too big to be man-made. The photo above shows a massive ice circle in Lake Baikal, Russia. The circle is over 4 kilometers wide, and the scientific explanation for its occurrence is described in easy to understand language on their site.
While you are there, be sure to check out some of the other amazing imagery of our planet published by NASA on their Earth Observatory site.