Permafrost is soil that has remained at or below the freezing point for two or more years. The term is actually a bit misleading, as no frost is required. Permanently frozen bedrock, for example, can also be referred to as permafrost, as can very dry frozen soil. Between 20-25% of the Earth's surface is covered in permafrost — also known as cryotic soil — although this number began to fluctuate wildly in the early 21st century due to global warming.
For soil to get this way, the climate has to be extremely cold. Most of the time soil in this condition closely mirrors air temperature, and is found in the polar regions of the world. Alpine permafrost is found in mountainous areas in regions of lower latitude, and other similar can be found spotted across the Earth's surface. In some cases, permafrost is hundreds of thousands of years old, in which case it is considered to be “fossilized,” accumulated over a period of time which took thousands of years and impossible to melt in current climate conditions.
As one might imagine, cryotic soil has a radical impact on the regions where it is found. It inhibits plant growth which makes it difficult for animals to thrive. It also helps resist erosion, as it essentially cements the soil together, and it presents a number of challenges to people. Building on permafrost is tricky, because if it melts then the structures built on top of it can collapse. Likewise, utility pipes in regions with cryotic soil must be run above the ground for safety, as they could melt the soil and cause a problem, or they could crack open if the soil melted and the ground settled.
Researchers are especially interested in the areas of “continuous permafrost,” where cryotic soil is extensive and it has been present for an extended period of time. This often creates distinctive patterns in the ground which can be interesting to observe, and the spread or shrinking of the soil can be an indicator of environmental problems. In Arctic nations, for example, the shrinking permafrost line has contributed substantially to erosion.
The permafrost regions of the world are also of interest to archaeologists and historians. Frost is an excellent preservative, literally freezing artifacts in time. In addition to finding human and animal remains in cryotic soil, researchers have also found organic materials like textiles, leather, and baskets which would normally decay.