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Significant numbers of viable ancient microorganisms are known to be present within the permafrost. They have been isolated in both polar regions from the cores up to 400 m deep and ground temperatures of −27°C. The age of the cells corresponds to the longevity of the permanently frozen state of the soils, with the oldest cells dating back to ~3 million years in the Arctic, and ~5 million years in the Antarctic. They are the only life forms known to have retained viability over geological time. Thawing of the permafrost renews their physiological activity and exposes ancient life to modern ecosystems. Thus, the permafrost represents a stable and unique physicochemical complex, which maintains life incomparably longer than any other known habitats. If we take into account the depth of the permafrost layers, it is easy to conclude that they contain a total microbial biomass many times higher than that of the soil cover. This great mass of viable matter is peculiar to permafrost only. [ABSTRACT FROM AUTHOR]