A beach in Finland appeared to be covered in thousands of eggs this week, but an up-close look shows those white spheres are not shells, they are actually made out of ice. An amateur photographer came across the "ice eggs" on Hailuoto Island, located between Finland and Sweden.

The photographer, Risto Mattila, told BBC he has never seen anything like it before — and experts say the "ice eggs" are actually rare. The phenomenon is caused when small pieces of ice are rolled over by wind and water, according to BBC News.

"That was an amazing view. I have never seen anything like this during 25 years living in the vicinity," he said.

Get Breaking News Delivered to Your Inbox

While the smallest of the ice balls were the size of eggs, the biggest ones were the size of footballs — and they can get even bigger, up to 3 feet, BBC News reports. According to Mattila, the eggs covered about 100 feet of beach near the waterline.

Risto Mattila and his wife were walking on the beach of a Finnish island when they came across thousands of "ice eggs." Risto Mattila

Mattila said he went to the island on a cold, windy day, which is prime conditions for this phenomenon, according to BBC Weather expert George Goodfellow. "The general picture is that they form from pieces of larger ice sheet which then get jostled around by waves, making them rounder," he said.

"They can grow when sea water freezes on to their surfaces and this also helps to make them smoother," Goodfellow added. "So the result is a ball of smooth ice which can then get deposited on to a beach, either blown there or getting left there when the tide goes out."

Similar "ice eggs" have been spotted in Russia and even on Lake Michigan in Chicago.