Permafrost

Permafrost, perennially frozen layer of soil and water that occurs in tundra regions in arctic, subarctic, and alpine areas. Permafrost lies below the surface and forms where the temperature has continuously been below freezing for at least two years. It is found under about one fifth of the Earth’s land area. Vast tracts of permafrost lie across Canada, Alaska, northern Europe and Asia, and Antarctica. Greenland is almost totally covered with permafrost. Pockets of permafrost are found as far south as the 50th parallel in Canada and the 45th parallel in Siberia. Clues to the age of the permafrost of the Northern hemisphere lie in the numerous discoveries of remains, embedded in frozen ground, of mammoths, which became extinct about 10,000 to 15,000 years ago—coincident with the end of the most recent ice age.