When an area of ground has remained below the freezing point for two or more years, it is said to be in a state of permafrost. The term “permafrost” is actually a bit misleading, as no frost is required. Permanently frozen bedrock, for example, is in a state of permafrost, as is extremely dry frozen soil. 20-25% of the Earth's surface is covered in permafrost, depending on which source one relies upon, although this number began to fluctuate wildly in the early 21st century due to global warming.

In order for permafrost to exist, the climate has to be extremely cold. Most permafrost 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 areas of permafrost can be found spotted across the Earth's surface. In some cases, permafrost is hundreds of thousands of years old, and in fact such permafrost can be “fossilized,” accumulated over a period of time which took thousands of years and impossible to melt in current climate conditions due to the extent of the permafrost.

As one might imagine, permafrost has a radical impact on the regions where it is found. Plant growth is inhibited in areas of permafrost, making it difficult for plants and, by extension, animals, to thrive. Permafrost also helps to resist erosion, as it essentially cements the soil together, and it presents a number of challenges to people. Building on permafrost is tricky, as if the permafrost melts, buildings can subside. Utility pipes must be run above ground in regions of permafrost for safety, as they could melt the permafrost and cause a problem, or they could crack open if the permafrost melted and the ground settled.

Researchers are especially interested in the areas of “continuous permafrost,” where permafrost is extensive and it has been present for an extended period of time. Continuous permafrost often creates distinctive patterns in the ground which can be interesting to observe, and the spread or shrinking of continuous permafrost 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 permafrost, researchers have also found organic materials like textiles, leather, and baskets which would normally decay.