The flat land of the Mackenzie River delta is dotted with small, cone-shaped hills with hearts of ice. Called 'pingoes,' these low mounds are the result of frozen ground, called permafrost, being forced upward by the pressure of subterranean water.

Tuktoyaktuk boasts more than 1,400 pingos. One is famous enough to have a name - 'Ilyuk.' It is estimated to be 1,000 years old and still growing.

Hollow pingoes are used by Inuit hunters as meat freezers.

Permafrost is defined as ground that has remained frozen for at least two years. It forms where the winters are long and frigid and the ground is without a thick insulating layer of snow.

Permafrost is a serious obstacle to economic development in the Mackenzie River valley. Seasonal freezing and thawing makes much of the terrain unstable and, therefore, unsuitable for the building of tall buildings. Farming is impossible.

Drilling for oil and gas, mining, and construction of hydro dams are much more difficult tasks than in the South. Paved roads and runways need to be insulated to prevent them from melting the permafrost beneath.

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