pingo, dome-shaped hill formed in a permafrost area when the pressure of freezing groundwater pushes up a layer of frozen ground. Pingos may be up to 90 metres (300 feet) high and more than 800 metres (0.5 mile) across and are usually circular or oval-shaped. The core, which may be only slightly smaller than the pingo itself, consists of a mass of clear ice. Modern pingos occur in the continental tundras and are generally restricted to latitudes of 65° to 75° N. Rupture of the overlying material at the top of the pingo exposes the ice to melting and may