A layer of soil or bedrock at a variable depth beneath the surface of the
earth in which the temperature has been below freezing continuously from a few
to several thousands of years.
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While the top layer of soil heats up in the summer, permafrost exists in
deeper layers where the summer heating fails to melt the frozen ground. A
continuous layer of permafrost is found in areas where the annual mean
temperature is below about −5°C (23°F).
