An ice disc 55 feet across has been spotted swirling round and round atop the water of North Dakota's Sheyenne River.
According to the Associated Press, record-setting air pressure combined with subfreezing temperatures to turn the river water to ice – but not all at once. When the first chunks of floating ice took shape, they got caught in the river's eddy and started to spin in a circle, collecting into the disc seen here. Here's a video, shot in the UK in 2010, that appears to show one such disc coming into being:
"It's not a continuous sheet of ice," said Allen Schlag, a National Weather Service hydrologist in Bismarck, ND. "If you were to throw a grapefruit-size rock on it, it would go through. It's not a solid piece of ice — it's a collection of ice cubes."
Frozen ice-discs aren't unheard of (searching YouTube turns up several examples, including the UK disc above, or this one, which formed in Vermont in 2010), but they are rare.
"That might be one of the better examples I've seen," said Schlag, noting the disc's size (he says big discs like this one are more common on bigger rivers). "It's a pretty cool one."
More weird ice phenomena here and here