What is a Pingo?

Pingo is an Inuit (Northern Eskimo) word meaning hill. The term was borrowed in 1938 by Danish botanist, Alf Erling Porsild, to describe a specific kind of hill: a rounded or conical hill having an ice core, and only found in regions with permafrost (permanently frozen soils). Even in permafrost areas there is an active top layer of soil that thaws during the summer and refreezes each winter. Beneath rivers and lakes thaw bulbs of unfrozen ground form that can stay thawed even in winter.

Ibyuk Pingo, NWT, Cananda - Source: Helen Kerfoot, NRC
The most common pingos form in the saturated soils of alluvial deltas where lakes
have either dried up or the topography has shifted allowing the lake to drain. In such a
case the top layer of soil then refreezes, trapping water-saturated soil underneath.
When water is pressurized the temperature at which it will freeze decreases, so the
water underneath the newly frozen ground can remain liquid. If the layer of ground
over the water-saturated soil does not thaw the next summer, and the freezing
continues for several years, the pressure on the trapped water increases, eventually
forcing the water up and creating a bubble undeneath the top vegetative mat. This
bubble of expanded water then freezes, forming the core of a newly created pingo.

A pingo can grow over the years, eventually reaching up to 200 feet in height with a
diameter over 900 feet. In the pingo's mature stages cracks can form in the summit,
exposing the ice core. Some of the ice then melts, forming a small fresh water lake.
These pingo lakes are among the few guaranteed sources of fresh water in areas such
as the Arctic Coastal Plain.