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Permafrost is defined as soil or rock material, with or without included moisture or organic matter, that has remained at or below 0°C for two or more years (Muller, 1945, p. 3). It is defined exclusively on the basis of temperature; however, one of the most important properties of permafrost is the amount of ice it contains. Permafrost with little or no ice generally does not cause engineering or environmental problems, but permafrost that is ice rich can cause extremely serious problems if allowed to thaw. Ice in active glaciers is not considered permafrost even though it fits the general definition of permafrost.

Permafrost underlies approximately 20 percent of the world’s land mass (Muller, 1945, p. 3), and most of it is present in the Northern Hemisphere because the land area is much greater there than it is in the Southern Hemisphere (Fig. 1). However, permafrost is widespread in Antarctica. In addition to Alaska, where 85 percent of the land is within the permafrost region, extensive areas of permafrost are present in other countries in the Northern Hemisphere. These are Canada (50 percent), the U.S.S.R. (50 percent), and the People’s Republic of China (20 percent).

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