While hunting in North Dakota, retired engineer George Loegering came upon a circle of ice over 50 feet in diameter gently revolving on the surface of the Sheyenne River.
Leogering took photos and video of the phenomenon, which looks like something environmental artist Andy Goldsworthy might have created while no one was watching.
But it's not artificial. A combination of cold, dense air and an eddy in the river likely caused the formation, Allen Schlag, a National Weather Service hydrologist, and Greg Gust, a weather service meteorologist, told The Telegraph.
Air pressure in the nearby city of Fargo had reached a record high on Saturday for the month of November, says Gust, adding that the river was fairly warm, so the cold air turned the water to ice only sporadically. Bits of ice floating along got caught up in the eddy, began spinning in a circle, and formed the final result: