How do pingos form in permafrost landscapes?

A pingo is a dome-shaped hill found in permafrost landscapes that forms when the pressure of freezing groundwater pushes up a layer of frozen ground. These features are categorized into two types based on their formation process:12

• Open-system pingos: These form in areas of thin or discontinuous permafrost where liquid water is supplied by a groundwater aquifer. Artesian pressure forces water toward the surface, where it freezes into a lens shape and pushes the overlying material upward.1
• Closed-system pingos: These occur in flat areas with limited groundwater, such as river deltas. They form when advancing permafrost creates upward pressure on a confined mass of saturated soil, causing it to freeze and expand, which lifts the ground above it.1

The core of a pingo typically consists of a mass of clear ice. If the surface material ruptures, the exposed ice may melt, often resulting in the formation of a crater or lake.1