Lots of surprising things wash up on the world’s beaches—including snowballs. As NPR reports, a beach in northwest Siberia, on the Gulf of Ob, is covered in frozen spheres that have drifted ashore. According to experts, the rare phenomenon occurs when pieces of ice form, and are rolled into balls by wind and water.

The frosty orbs began accumulating along the Siberian coast around two weeks ago, according to the BBC. Currently, they cover around 11 miles of beach. Some are the size of a tennis ball; others are nearly 3 feet across.

Sergei Lisenkov, press secretary of the Arctic and Antarctic Research Institute, provided Russian media outlets with an explanation. “As a rule, first there is a primary natural phenomenon—sludge ice, slob ice,” he said. (NPR defines “slob ice” as a Canadian term used to describe "sludgy masses of floating ice.”)

“Then comes a combination of the effects of the wind, the lay of the coastline, and the temperature and wind conditions,” Lisenkov continued. “It can be such an original combination that it results in the formation of balls like these.”

Residents of the nearby village of Nyda are amazed. "Even old-timers say they see this phenomenon for the first time," village administrator Valery Akulov told The Siberian Times.

But as other media outlets point out, this isn’t the first time that waves have washed spherical ice formations onto land: Similar events have been reported in the U.S. around Lake Michigan, and in the Gulf of Finland in Europe.

In short, the unusual weather surprise is rare, but perfectly natural. Hopefully, the people of Nyda love a good snowball fight—especially one that comes complete with ready-made ammo. Watch a video of the wintry occurrence below.

[h/t NPR]