Permafrost is soil, rock or sediment that remains below 0 centigrades for two or more consecutive years.
Permafrost underlies most of the Arctic land area. In parts of Siberia, it lies up to 1500 m thick. Within permafrost, water is frozen and ties together the soil, rock or sediment. This impermeable layer of ice below the surface shapes the landscape. It prevents water from draining away in summer, leading to high water levels and wet conditions in many areas. It has also provided a solid foundation on which to build, ever since humans first inhabited the Arctic.
There is permafrost beneath the Arctic Ocean too. It is called subsea permafrost and mostly occurs where land was inundated by ocean at the end of the last ice age, 10 000 years ago. Its thawing could lead to large releases of methane to the atmosphere.