A rare collection of “ice eggs” has been spotted in Finland, a phenomenon experts say only occurs in highly particular conditions.

Risto Mattila, who photographed the eggs, said he and his wife were walking along Marjaniemi beach on Hailuoto island on Sunday when they came across the icy balls covering a 30-meter (98ft) expanse of shoreline.

“The biggest of the eggs was about the size of a football,” said Mattila, an amateur photographer. “It was an amazing view. I have never seen this phenomenon before.”

Jouni Vainio, an ice specialist at the Finnish Meteorological Institute, said the occurrence was not common, but could happen about once a year in the right weather conditions.

“You need the right air temperature (below zero, but only a bit), the right water temperature (near freezing point), a shallow and gently sloping sandy beach and calm waves, maybe a light swell,” he said.

Read more: The Guardian