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Permafrost is soil or underwater sediment which continuously remains below 0 C for two years or more; the oldest permafrost has been continuously frozen for around 700,000 years. Whilst the shallowest permafrost has a vertical extent of below a meter (3 ft), the deepest is greater than 1500 m. Similarly, the area of individual permafrost zones may be limited to narrow mountain summits or extend across vast Arctic

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