Thermokarst is a land surface characterised by very irregular surfaces of marshy hollows and small hummocks formed as ice-rich permafrost thaws, that occurs in Arctic areas, and on a smaller scale in mountainous areas such as the Himalayas and the Swiss Alps. These pitted surfaces resemble those formed by solution in some karst areas of limestone, which is how they came to have karst attached to their name without the presence of any limestone. Small domes that form on the surface due to frost heaving with the onset of winter are only temporary features. They then collapse with the arrival of next summer's thaw and leave a small surface depression. Some ice lenses grow and form larger surface hummocks, which last many years and sometimes become covered with grasses and sedges, until they begin to thaw. These domed surfaces eventually collapse either annually or after longer periods and form depressions which contribute to uneven surfaces. These are included within the general label of thermokarst.
The Batagaika crater in Siberia is an example of a large thermokarst depression.
The related term thermokarst lake, also called a thaw lake or cave-in lake, refers to a body of freshwater, usually shallow, that is formed in a depression by meltwater from thawing permafrost. Depressions are often produced by the collapse of ground levels associated with permafrost thaw. Continued thawing of the permafrost substrate can lead to the drainage and eventual disappearance of thermokarst lakes, leaving them, in such cases, a geomorphologically temporary phenomenon. In recent years, thermokarst lakes have become increasingly common in Siberia and other tundra environments.
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