A landscape pattern formed by the melting of permafrost and characterized by patterns of hummocks and water-filled depressions.
The Batagaika crater in the Siberian taiga, an example of a thermokarst, is sometimes called the "doorway to the underworld":
The crater began when a section of forest was cleared in the 1960s. With the ground no longer shaded, the permafrost began to melt, causing the ground to collapse; subsequent flooding helped to continue the process. Here is an article about the crater.
Etymology: The Karst is a limestone plateau near Trieste; the word 'karst', denoting an area of irregular limestone characterized by sinkholes and underground streams, derives from that. 'Thermokarst' refers to a karst-like (there might not be any limestone) formation that is the result of thawing (heat).