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A pingo is a curious landform found in tundra zones and around the edges of Glaciers. It's a little hill with a core of ice.

Pingoes generally form at a depression in the permafrost level below ground, as a result of repeated freezing and thawing. During the summer, liquid water collects in this depression. In the winter, the water freezes and the ice expands, pushing everything above it up. As the years pass, a chunk of ice gradually grows above the depression; at the surface, a sort of hill rises from the ground.

                          _-   _____   -_
Ground Level ->    -------   _-     -_   -----------
                         __--         --__
                  ____---       Ice       ---____
Permafrost Level ->------___           ___-----------
           Water collects here
           during the summer.

Pingoes will even form underwater, if there is a permafrost layer beneath the sea bottom.

More Glacier terminology can be found by following the link.

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