A Finnish couple discovered thousands of “ice eggs” while walking along a frigid beach on Sunday, a rare phenomenon that experts say only occurs in unique conditions.

Amateur photographer Risto Mattila from Oulu, Finland, posted the images to his Twitter account, showing thousands of spheres made of ice spread across the shoreline of Marjaniemi beach. In his image caption, Mattila refers to the rarity as “snow ball sea.”

Mattila said that he stumbled upon the icy wonder while walking along the beach with his wife. He said the balls, which he referred to as “ice eggs” covered 30 meters (98 feet) of the shoreline. The largest one, he said, was about the size of a soccer ball.