thermokarst, land surface configuration that results from the melting of ground ice in a region underlain by permafrost. In areas that have appreciable amounts of ice, when the ice melts and the ground settles unevenly, small pits, mounds, sinkholes, tunnels, caverns, valleys, steep-walled ravines, and hummocks are formed. The hummocky ground surface resembles the karst topography of limestone areas.

The size and form of the features depends on the nature and extent of the ice. In general, regional thermokarst activity indicates an overall warming of the climate, whereas localized thermokarst landforms may be caused by the movement of heavy vehicles or the construction of buildings over permafrost, the introduction of agriculture, or deforestation. Individual features have no characteristics that are exclusively due to thermokarst activity, and they are hard to distinguish from those formed by other means. Features resulting from the melting of dead glacial ice are not classified as thermokarst forms.