Pingos are huge landforms, roughly 500m in diameter and 50m in height.  Pingos are often located in Canada and Greenland.  There are two types of pingos:

• Closed system pingos are known as Canada or Mackenzie pingos due to their location.  Canada pingos occur in areas of continuous permafrost.  On the site of a lake, groundwater is trapper by freezing from above and permafrost below expanding inwards.  The water freezes and expands, pushing overlying sediment upwards.  If the pressure is too great, the top of the sediment heap can rupture and collapse.  It may then infill with water.
• Open system pingos are known as Greenland pingos and occur in areas of discontinuous permafrost.  Water infiltrates from the surface to the layer below during the summer.  Freezing and expansion cycles force sediment up into a dome shape which can rupture if the pressure becomes too great.

When a pingo ruptures, the collapsed structure is known as an ognip.