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Thousands of egg-shaped balls of ice have covered a beach in Finland, the result of a rare weather phenomenon.

Amateur photographer Risto Mattila was among those who came across the "ice eggs" on Hailuoto Island in the Gulf of Bothnia between Finland and Sweden.

Experts say it is caused by a rare process in which small pieces of ice are rolled over by wind and water.

Mr Mattila, from the nearby city of Oulu, told the BBC he had never seen anything like it before.

"I was with my wife at Marjaniemi beach. The weather was sunny, about -1C (32F) and it was quite a windy day," he told the BBC.

"There we found this amazing phenomenon. There was snow and ice eggs along the beach near the water line."

Mr Mattila said the balls of ice covered an area of about 30m (100ft). The smallest were the size of eggs and the biggest were the size of footballs.

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

"Since I had a camera with me I decided to preserve this unusual sight for posterity."

BBC Weather expert George Goodfellow said conditions needed to be cold and a bit windy for the ice balls to form.

"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. 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 sights have been reported before, including in Russia and on Lake Michigan near Chicago.