Pingos are distinctive geological formations found in periglacial landscapes, which are regions adjacent to glaciers or ice sheets. Essentially, pingos are large mounds or hills that can reach heights of up to $70$ metres and diameters of $600$ metres. Their formation is closely associated with the freezing and expansion of subterranean water, a process driven by the extremely low temperatures characteristic of these environments.

The formation process begins when a body of underground water starts to freeze. As water freezes, it undergoes expansion, which exerts pressure on the surrounding soil and rock, causing the ground to rise. Over time, this upward movement leads to the creation of a mound or hill, known as a pingo. The core of a pingo consists of a significant mass of ice, while its outer layer is composed of soil and rock that has been displaced from the ground beneath.

There are two primary types of pingos: closed-system pingos and open-system pingos.

Closed-system pingos develop in areas where the ground is permanently frozen, known as permafrost. In these cases, the water source typically originates from a buried body of unfrozen ground, referred to as a talik. As the talik freezes and expands, it exerts enough pressure to push up the overlying permafrost and soil, resulting in the formation of a pingo.

Open-system pingos, in contrast, arise in areas with a continuous influx of groundwater. In these environments, water seeps into the permafrost, where it freezes and expands to create a pingo. Open-system pingos tend to be larger than their closed-system counterparts because the constant supply of water allows them to continue growing as long as the conditions remain favorable.

In summary, the formation of pingos in periglacial landscapes is a remarkable process that involves the freezing and expansion of subterranean water. This natural phenomenon leads to the development of large mounds or hills, which are characteristic features of these cold, icy environments.