What is permafrost?

Permafrost is a permanently frozen layer on or under Earth's surface. It consists of soil, gravel, and sand, usually bound together by ice. Permafrost usually remains at or below 0°C (32ºF) for at least two years. Permafrost can be found on land and below the ocean floor. It is found in areas where temperatures rarely rise above freezing. This means frozen soil is often found in Arctic regions such as Greenland, the U.S. state of Alaska, Russia, China, and Eastern Europe.

Source: National Geographic