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 permafrost is often found in Arctic regions such as Greenland, the U.S. state of Alaska, Russia, China, and Eastern Europe.

Permafrost thickness can range from one meter (about three feet) to more than 1,000 meters (about 3,281 feet). Permafrost covers approximately 22.8 million square kilometers (about 8.8 million square miles) in Earth's Northern Hemisphere. Frozen ground is not always the same as permafrost. A layer of soil that freezes for more than 15 days per year is called "seasonally frozen ground." A layer of soil that freezes between one and 15 days a year is called "intermittently frozen ground." Permafrost is frozen for two years or more.

Permafrost does not always form in one solid sheet. There are two major ways to describe its distribution: continuous and discontinuous.

Continuous permafrost is just what it sounds like: a continuous sheet of frozen material. Continuous permafrost extends under all surfaces except large bodies of water in the area. The part of Russia known as Siberia has continuous permafrost.

Discontinuous permafrost is broken up into separate areas. Some permafrost, in the shadow of a mountain or thick vegetation, stays all year. In other areas of discontinuous permafrost, the summer sun thaws the permafrost for several weeks or months. The land near the southern shore of Hudson Bay, Canada, has discontinuous permafrost.

Scientists who study permafrost are able to understand changes in Earth's climate by observing changes in permafrost. Studies show Earth's permafrost warmed by 6ºC during the 20th century. Scientists predict widespread thawing of permafrost by 2100.

When the temperature of permafrost rises above 0ºC, it may thaw, in which case any ice it contains will undergo a phase change from solid to liquid (i.e., it will melt). Generally, the mineral and organic components of the former permafrost will remain solid. Therefore, in regions with more ground ice, thawing permafrost may result in more of a change to the land surface, and vice versa.

Thawing permafrost can raise water levels in Earth's oceans and increase erosion. Erosion happens when permafrost thaws because soil and sediment are easily washed away without ice binding them together.