Cold, dense air and a river’s eddy were the forces behind a rare, spinning ice circle found earlier this week.

First discovered on the Sheyenne River in Bismarck, N.D., by a 73-year-old hunter, the ice disc is actually made up of small, floating ice cubes and measures nearly 17 metres across.

Based on the circle’s size and visible growth rings, National Weather Service hydrologist Allen Schlag said it was especially unusual to find the natural phenomenon in such a small river, the Associated Press reported.

Ice circles are most commonly found in North American rivers but have been found in parts of Europe.

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