AbstractSeven bogs adjacent to Puolejokk in Padjelanta, Swedish Lappland, contain palsas either in groups as low winding ridges, or as large isolated oval-shaped hummocks well above the surface of the bogs. In midsummer 1963, trenches 2 m long, 30 cm wide, were excavated down to the tjäle across several palsas in each of the four largest bogs. The thawed dark brown to dark reddish brown peat of each palsa rested on a frozen core of gray sand or of till. By mid-summer 1967, the previously trenched palsas had caved in, collapsed, or sunk beneath the water of the bogs.Permanently frozen ground exists under better drained nonsorted polygon areas nearby, indicating a climate conducive for palsa maintenance, although the larger palsas may have reached a growth stage such that any slight disturbance might produce deterioration. Alternatively, larger undisturbed palsas may be collapsing in the wetter bogs due to water level changes.