Permafrost

Permafrost, ground that remains at or below 0°C for at least two years, covers vast regions of the Arctic and high mountains. It ranges from less than a meter to over 1,500 m deep. Permafrost lies beneath an active layer that freezes and thaws seasonally, and is distinct from the ground under glaciers and ice sheets. The oldest known permafrost has been frozen for approximately 700,000 years.