Floating boulders of ice the size of basketballs lined the shores of Lake Michigan last month, and were captured in a photo.
Weighing in at up to 50 pounds (22 kilograms) each, the ice spheres are a winter weather phenomenon resulting from wind and wave action along the shore, according to reporting by NASA's Earth Science Picture of the Day. Small fragments of floating ice act like seeds, with layers upon layers of supercooled lake water freezing around them as the balls churn in the waves. Wind then pushes the ice concretions onshore.
The photo, snapped on the Sleeping Bear Dunes National Lakeshore along northeastern Lake Michigan, shows enormous ice spheres lining the beach on Feb. 21.
The ice balls turn up on beaches during the winter with some regularity, but they rarely reach this size, according to Tom Ulrich, deputy superintendent for the park, the NASA site reported.