A spinning ice disk spotted on the Sheyenne River in North Dakota is a totally natural phenomenon and not the work of aliens or secret government spies, according to reports.
Retired engineer George Loegering saw the giant frozen circle on Saturday (Nov. 23) while on a hunting trip with relatives, the Associated Press reported. About 55 feet (17 meters) around, the icy disk was spinning in the river current like a record on a turntable.
The massive pancake-shaped ice pans often turn up on flowing rivers in cold climates. Video and photos posted online show similar disks discovered in Canada, England and Sweden during winter.
Theories abound to explain their formation. National Weather Service forecasters told the Associated Press that the Sheyenne giant likely appeared because cold, dense air slowly froze the river surface in bits and pieces. The floating ice chunks were trapped in a river eddy, creating the rotating circle discovered by Loegering. In 1993, MIT researchers who sought to explain smaller ice swirls on Boston's Charles River also suggested current-driven eddies.
By Tuesday, the twirling had ground to halt but the circular shape of the disk was still visible in the river, Loegering said. "I'm not sure how long it was there (spinning)," he told the AP. "It had to be quite a long time. If you look at the picture, you can see growth rings on the disk."
Email Becky Oskin or follow her @beckyoskin. Follow us @livescience, Facebook & Google+. Original article on LiveScience.