Pingos are intrapermafrost ice-cored hills, 3–70 m (10–230 ft) high and 30–1,000 m (98–3,281 ft) in diameter. They are typically conical in shape and grow and persist only in permafrost environments, such as the Arctic and subarctic. A pingo is a periglacial landform, which is defined as a non-glacial landform or process linked to colder climates. It is estimated that there are more than 11,000 pingos on Earth, with the Tuktoyaktuk peninsula area having the greatest concentration at a total of 1,350.

In 1825, John Franklin made the earliest description of a pingo when he climbed a small pingo on Ellice Island in the Mackenzie Delta. However, it was in 1938 that the term pingo was first borrowed from the Inuvialuit by the Arctic botanist Alf Erling Porsild in his paper on Earth mounds of the western Arctic coast of Canada and Alaska. Porsild Pingo in Tuktoyaktuk is named in his honour. The term pingo, which in Inuvialuktun means conical hill, has now been accepted as a scientific term in English-language literature.

Pingos can only form in a permafrost environment. Evidence of collapsed pingos in an area suggests that there was once permafrost. Pingos can collapse due to the melting of the supporting ice and give rise to a depression in the landscape showing an inverse shape (horizontal mirror).

Closed systems, also known as hydrostatic pingos, are formed as a result of hydrostatic pressure that has built up within the core of pingos due to water. They occur in regions of continuous permafrost where there is an impermeable ground layer. These pingos are found in flat, poorly drained areas with limited groundwater available such as shallow lakes and river deltas. The formation of these landforms occurs when layers of permafrost generate an upwards movement or pressure, resulting in masses of confined soil freezing, which pushes material upwards due to expansion.

The figure illustrates this process and the changes that occur throughout the year. This type of closed system pingos is formed in an area where a lake has been infilled with sediment. This indicates that the ground is insulated, allowing liquid water to collect underneath the sediment. In winter months this sediment begins to freeze which leads to expansion of sediment, confining the water and increasing the pressure. This results in the formation of a mound due to the upwards pressure. However, during summer months the ice core of the pingo begins to melt which causes the mound to cave in.

Hydraulic (open-system) pingos result from groundwater flowing from an outside source, i.e. sub-permafrost or intra-permafrost aquifers. Hydrostatic pressure initializes the formation of the ice core as water is pushed up and subsequently freezes. Open-system pingos have no limitations to the amount of water available unless the aquifers freeze. They often occur at the base of slopes and are commonly known as Greenland type. The groundwater is put under artesian pressure and forces the ground up as it makes an expanding ice core. It is not the artesian pressure itself that forces the ground up, but rather the ice core that is being fed the water from the aquifer. These are often formed in a thin, discontinuous permafrost. These conditions allow an ice core to form, but also provide it with a supply of artesian ground water. If water pressure entering an artesian pingo is strong enough, it can lift the pingo up allowing a sub-pingo water lens to form underneath. However, if the water lens starts to leak water it can cause subsidence which can compromise the structure. These pingos are often oval or oblong shaped. It is still not entirely understood why open system or hydraulic pingos normally occur in unglaciated terrain.

Pingos usually grow only a couple of centimetres per year, with Ibyuk Pingo growing at a rate of 2 cm (0.79 in) a year, and the largest take decades or even centuries to form. The process that creates pingos is believed to be closely related to frost heaving. The base of the pingo tends to reach its maximum diameter in its early youth. This means pingos tend to grow higher rather than growing in diameter and height at the same time. The height of pingos can range anywhere from 3 to 70 m (9.8 to 229.7 ft) and their diameters range from 30 to 1,000 m (98 to 3,281 ft). The shape of pingos is usually circular. Smaller pingos tend to have curved tops whereas larger pingos usually have collapsed mounds or craters due to the melting of exposed ice.

The landscape of Greenland contains many pingos and other glacial landforms. In western Greenland it is estimated that there are 29 pingos, whilst in eastern Greenland it is estimated there are 71 pingos. The majority of pingos in Greenland are located within Disko Bay and Nuussuaq Peninsula within western Greenland as well as some in eastern Greenland in Mesters Vig. The permafrost at Disko Bay is around 150 m (490 ft) deep, providing for ideal conditions for the development of closed system pingos. There are 20 pingos located on Disko Island, with the largest located on Kuganguaq alluvial plain at 100 m (330 ft) wide and 15 m (49 ft) high.