Areas of rock and soil where temperatures have been below freezing point for at least two years. Permafrost need contain no ice; a sub-zero temperature is the sole qualification. Continuous permafrost is present in all periglacial areas apart from small, localized thawed zones, while discontinuous permafrost exists as small, scattered areas of permanently frozen ground. A frost table marks the upper limit of permafrost, which is overlain by the active layer. Ice develops mainly in upper levels in epigenetic permafrost; in syngenic permafrost it is regularly distributed throughout the whole thickness of the permafrost; go to the US Army Permafrost tunnel for both types.

Permafrost aggradation (growth) decreases the thickness of the active layer, is responsible for the formation of pingos, and may be caused by the freezing of taliks. Permafrost degradation (decline) plays a key role in the development of thermokarst; see Jorgensen et al. (2001) Clim. Change 48, 4. Permafrost is a very sensitive system; small mistakes in constructing buildings in this environment can have catastrophic effects; see Couture et al. (2000) Geol. Soc. Canada, 2000–B2.