AbstractThis paper supplements earlier reports on palsas and pingo-like frost mounds in northeasternmost Scandinavia (Fig. 1) and treats a form of circular to oval lakes having characteristics of both the palsa and the pingo form groups. The lakes differ from other lakes of regular shape by being enclosed by a low, uniform wall.The circular lakes are interpreted as a collapse stage of frost mounds, which may have originated as palsas or as pingos. Also, some localities with fossil circular forms have been observed, mostly situated at lower elevations.The climatic significance of the collapse features is briefly analyzed in comparison with that of the palsas. For the fossil, low-elevation rings, a climatological and temporal interpretation is advanced referring to radiocarbon dating and to the occurrence of fossil ice-wedge polygons in the area.