Thermokarst is a type of terrain characterised by very irregular surfaces of marshy hollows and small hummocks formed as ice-rich permafrost thaws. The land surface type occurs in Arctic areas, and on a smaller scale in mountainous areas such as the Himalayas and the Swiss Alps. These pitted surfaces resemble clusters of small lakes formed by dissolution of limestone in some karst areas, which is how they came to have "karst" attached to their name, even though no limestone is actually present. Small domes that form on the surface due to frost heaving with the onset of winter are only temporary features. They collapse during the following summer thaw, leaving a small surface depression. Some ice lenses grow and form larger surface hummocks ("pingos") which can last for 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 become part of the uneven terrains included under the general category of thermokarst. The formation of permafrost thaw lakes due to warming climate is a positive feedback loop, as methane, nitrous oxide and carbon dioxide are released as permafrost thaws, contributing to further climate warming. The Batagaika crater in Siberia is an example of a large thermokarst depression. Thermokarst lakes A thermokarst lake, also called a thaw lake, tundra lake, thaw depression, or tundra pond, refers to a body of freshwater, usually shallow, that is formed in a depression formed by thawing ice-rich permafrost. A key indicator of thermokarst lakes is the occurrence of excess ground ice as well as having an ice content with greater than 30% by volume. Thermokarst lakes tend to form and disappear in a cyclical manner, resulting in a predictable life cycle (see "life cycle" below). 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, formed in response to a warming climate. These lakes are typically found in arctic and subarctic lowlands, including the western Canadian Arctic (e.g. Banks Island, Victoria island), the Alaskan coastal plain, interior Yukon Territory and the alluvial lowlands of northern Eurasia and Siberia. The presence of thaw lakes in a region results in a thermal disturbance as the water warms the ground. The depth of permafrost below a lake will generally be shallower