Permafrost is ground that continuously remains below 0°C (32°F) for two or more years, located on land or under the ocean. Permafrost does not have to be the first layer that is on the ground. It can be from an inch to several miles deep under the Earth's surface. Some of the most common permafrost locations are in the Northern Hemisphere. Almost a quarter of the Northern Hemisphere is underlain by permafrost, including 85% of Alaska, Greenland, Canada and Siberia. It can also be located on mountaintops in the Southern Hemisphere. Permafrost frequently occurs in ground ice, but it can also be present in non-porous bedrock. Permafrost is formed from ice holding various types of soil, sand, and rock in combination.
