Preparing for an epic snowball fight this winter? The best place to stock up on ammo may be a beach in Siberia, where thousands of huge, perfectly round snowballs are piling up, according to news reports. But where are these frozen orbs coming from?

Villagers near the Gulf of Ob in Siberia discovered the snowballs along an 11-mile stretch of the beach, reported the Siberian Times. The snowballs range from the size of a tennis ball (about 2.7 inches) to almost 3 feet across.

Anyone for a snowball fight? Nature makes its own collection of snowballs on the Arctic shore of the Gulf of Ob https://t.co/sona6DeNr4 pic.twitter.com/9gtG9foJKb — The Siberian Times (@siberian_times) November 4, 2016

Though they look strange, the orbs are naturally occurring, experts say. [Images: One-of-a-Kind Places on Earth]

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“It’s a rare natural phenomenon,” Sergey Lisenkov, a spokesperson for the Arctic and Antarctic Research Institute (AARI), told the Siberian Times. “As a rule, grease ice forms first, slush. And then a combination of the action of the wind, the outlines of the coastline, and the temperature may lead to the formation of such balls.”

According to news reports, the snowballs first formed in late October, after water in the Gulf of Ob rose and covered the beach in ice. Just as kids roll snowballs along a snow-covered surface to create bigger spherical creations, ice on the beach rolled along the sand as the tides receded, creating the frozen orbs.

Area residents said the phenomenon was a surprise, and had not happened previously.

“Even old-timers say they see this phenomenon for the first time,” Valery Akulov, from the village administration, told the Siberian Times.

A similar phenomenon has occurred along Lake Michigan, where boulder-size ice balls can form in winter months. When chunks of the ice sheets that cover parts of the lake in winter break off, they churn in the waves and become ice spheres.

Snow rollers are another form of naturally occurring snowballs that can invade during winter months. Snow rollers occur only in the right conditions: a combination of light, sticky snow; strong (but not too strong) winds; and cold temperatures, according to the National Weather Service. When snow-covered landscapes are blasted by blustery winds, the snow can be sculpted into doughnuts, hollow tubes and snowballs.

Original article on Live Science.