Permanently frozen subsoil with a temperature below 0 C (32 F) continuously for two or more years. The permafrost line is a line demarcating regions where the subsoil is permanently frozen. Permafrost is overlain by a surface layer that is subject to thawing during the warmer seasons of the year. This zone of seasonal freezing and thawing is termed the active layer. Permafrost is related to the tree line, because the frozen ground prevents tree roots from penetrating deeply and inhibits the subsurface drainage of meltwater. Permafrost is estimated to underlie 20 percent of the Earth's land surface and reaches depths of 1,500 meters (5,000 feet) in northern Siberia. It occurs in 85 percent of Alaska, more than half of Russia and Canada, and probably all of Antarctica. Permafrost has preserved the carcasses of extinct Ice Age mammals; one or two almost complete, frozen mammoths dating from at least 10,000 years ago have been reported from Siberia.