Permafrost, a layer of permanently frozen ground in areas where temperatures are too low to permit complete thawing. Permafrost underlies most of the tundra region of North America and Eurasia, and extends southward beneath parts of the northern coniferous forest. Occurrence varies from continuous layers as thick as 5,000 feet (1,500 m) to isolated patches 5 feet (1.5 m) or less in depth.

In summer the ground surface above the permafrost thaws, creating boglike conditions. Summer travel is difficult in permafrost areas, and special construction techniques must be used for buildings and roads to prevent their damage or collapse when the ground surface thaws.