Nov. 7 (UPI) -- Amateur photographer Risto Mattila captured a picture of thousands of "ice eggs" covering a beach in Finland after a rare weather event over the weekend.
The egg-shaped balls were discovered along Marjaniemi beach on Hailuoto island in Finland.
They covered about 100 feet of shoreline, and ranged from the size of eggs to the size of a footballs, Mattilla, who is from nearby city of Oulu, told the BBC, adding that it was unlike anything he's ever seen before.
Matilla stumbled upon the ice eggs while he was walking along the beach with his wife Sunday.
"That was an amazing view," Matilla told the BBC. "I have never seen anything like this during 25 years living in the vicinity."
"Since I had a camera with me I decided to preserve this unusual sight for posterity," he added.
Dr. James Carter, Illinois State University emeritus professor of geography-geology, said the ice eggs are formed as slush is moved by waves.
"I can picture the back-and-forth motion of the surface shaping the slushy mix," Carter said. "Thanks to the photographer who shared the photos and observations, now the world gets to see something most of us would never be able to see."
Finnish Meteorological Institute ice specialist Jouni Vainio said the ice eggs are a rare occurrence, but could happen about once a year if the weather conditions are right.
"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. "You also need something that acts as the core. The core begins to collect ice around it and the swell moves it along the beach, forward and back. A small ball surface gets wet, freezes and becomes bigger and bigger."
Similar phenomena have been reported previously in Russia and Lake Michigan, near Chicago.
In 2016, residents of Nyda in Siberia discovered ice balls covering 11 miles of coastline, ranging from the size of a tennis ball to almost 3 feet across.