"permafrost." YourDictionary, n.d. Web. 06 December 2018. <https://www.yourdictionary.com/permafrost>.
permafrost. (n.d.). Retrieved December 06th, 2018, from https://www.yourdictionary.com/permafrost
A layer of soil or bedrock that has been continuously frozen for at least two years and as long as tens of thousands of years. Permafrost can reach depths of up to 1,524 m (999 ft). It is found throughout most of the polar regions and underlies about one fifth of the Earth's land surface.