From Wikipedia: Palsas are low, oval elevations occurring in polar climates. They develop over permafrost, where perennial ice lenses have developed within the soil. Like pingos, palsas consist of an ice core with overlying soil, but they are generally smaller than pingos, often occur in groups and may develop from ground water without additional hydrostatic pressure. Palsas are characteristically found in areas with discontinuous permafrost, and in such areas they may be the only reliable surface evidence of permafrost. Palsas need large quantities of water for the formation of their ice lenses, and for this reason they occur particularly in bogs.