"In freshwater ponds and lakes, the biological activity of microbes in the sediments on the lake floor produces bubbles of gas, usually methane or carbon dioxide. In winter this activity is slow, but it is still present.
The gas bubbles rise to the frozen surface of the lake, becoming trapped there. The following night, another layer of ice forms beneath the bubble, so it is encased in ice. This leads to the flattened shape you see. The picture is a frozen daily record of the gas emissions."
via new scientist