Eleven miles of Siberian coastline were covered in giant, smooth boulders of ice late last week, and they weren’t made by humans.

The orbs ranged from the size of tennis balls to yoga balls (three feet!), according to the BBC. “Locals in the village of Nyda, which lies on the Yamal Peninsula just above the Arctic Circle, say they have never seen anything to compare to them,” the BBC wrote.

This isn’t just a Siberia thing, though. Massive ice boulders wash up on the frigid shores of Lake Michigan and Lake Superior every decade or so. They tend to coincide with extreme cold outbreaks. That holds true in Siberia this year, too — Eurasian snow cover made a “huge advance” during October.

The phenomenon went viral in February 2013 when a woman posted a photos of snow orbs the size of beach balls on Sleeping Bear Dunes National Lakeshore on Lake Michigan. “It looks as if a bunch of giants are preparing for a snowball fight,” Michigan Live wrote in 2013.

The boulders form when smaller pieces of ice break off from the larger ice sheets on the lakes, according to Amie Lipscomb, a park ranger at Sleeping Bear Dunes. The waves churn these smaller pieces, and the edges are worn down and smoothed while new ice coats the sphere. It’s similar to the way hail forms.