This picture showing curious bubbles on the frozen Aar River was taken this past winter near Umiken, Aargau, Switzerland. It had been bitterly cold for several days prior to when the photo was snapped. Photographer Roger Benoit
It’s not known exactly how these bubbles formed, but it’s likely they were released from the riverbed and became trapped under a thin layer of ice. As deeper layers of water froze, perhaps additional bubbles became trapped further down in the ice — closer to the riverbed. Such a mechanism would also explain the flat appearance of individual inclusions. Typically, the faster the freezing rate, the greater the number and the size of the bubbles. In this case, the arrival of very cold air substantially increased the freezing rate. Note that the inside of one of the near-surface bubbles has been exposed (at right center). Photo taken on February 12, 2012.