soil frozen permanently (practically for a duration of at least two years)
In geology, permafrost is soil that stays frozen all year. To be called permafrost, soil must stay at or below 0 °C (32 °F) for two or more years. Permafrost is also called cryotic soil. Most permafrost is on land close to the North and South poles. Other permafrost is found in high mountain areas.  When permafrost melts, carbon dioxide is released which can be bad for the environment.
- ↑ Schuur, Ted (2016). "Thawing Permafrost Would Accelerate Global Warming. Thawing Arctic tundra will likely speed up climate change for a century or more. The question is: How drastically?". Scientific American. 315 (December 2016): 56–61. doi:10.1038/scientificamerican1216-56. PMID 28004690. Retrieved 25 November 2016.