A video that shows a perfect circle of ice slowly rotating in the Sheyenne River has amassed more than one million views in just over a week.
“It’s an amazing wonder,” George Loegering, a 73-year-old retired engineer, said in the video. “I don’t have a clue how it did it, but that thing is rotating.”
Despite Loegering’s bewilderment, scientists believe ice circles are formed under two specific weather conditions: a rapid drop in temperature and the presence of an eddy current (this simply refers to water flowing in a circular motion).
When a river quickly gets cold, small bits of ice form and usually flow straight downriver. But if they pass through an eddy current, they can get caught in its vortex. Then, if more ice chunks get sucked into the eddy, they sometimes stick together and become a circular mass of ice.
Ice circles might look smooth, but they are actually quite fragile, Allen Schlag, a National Weather Service hydrologist in North Dakota said to the AP. “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.”
Ice circles have also spotted around the world, including sightings in Moscow in 2010 and in Mississauga, Ontario, in 2008.
In fact, back in 1895, the magazine Scientific American published a letter to the editor that described an ice circle on the Mianus River in New York—referred to at the time as a “revolving ice cake.”