Permafrost - (n.)

A permanently frozen layer of soil, often hundreds of meters thick, found in the polar tundra. In the summer, the upper layer of the permafrost melts, resulting in muddy, swampy conditions. There is concern among climatologists that the increased temperature resulting from global warming will melt much of the Earth’s permafrost, with severe implications for soil stability and tundra ecology. See also “thermokarst.”