Permafrost can create a pingo when it sometimes acts as a barrier that doesn't allow groundwater to seep through. When a well is dug through large chunks of permafrost, for example, water sometimes rushes up and shoots out the well hole like a fountain. This phenomenon, called an artesian well, happens in low areas as a result of the pressure from groundwater in the surrounding hills. With its impermeable bulk, permafrost can hold water below the level gravity wants to push it. Interior pingos form when cracks in the permafrost allow water through.
Water moves away from the relatively warm earth, comes closer to cold air and freezes.
This process of freezing and expansion continues until a mound of earth is created-a pingo. Interior pingos often catch the eye because they attract different species of trees than the surrounding lowlands. Because pingo soil has been thrust upward by the ice below, the soil is often well-drained. Pingos often sport a cover of birch and aspen, while down below the ground wears a boggy coat of willows, alders and black spruce.