Hummock ice, Lake Peipus, Estonia

© Sven Zacek/Minden Picture

Lake Peipus, Estonia

Lake Peipus, the fifth largest lake in Europe, dates back hundreds of millions of years to the Paleozoic Era and is known for its sand dunes, which can “sing” when the wind blows just right. In the winter the frozen lake surface may feature ice hummocks, as seen in this image. The hummocks are caused by slow, uneven pressure in the ice pack.