Contained within the 4th Edition (1974) of the Atlas of Canada is a map that shows the area covered by permafrost across Canada. Permafrost is a term used to describe the thermal condition of earth materials, such as soil and rock, when their temperature remains below 0 degrees Celsius continuously for more than one year. The map also shows localities where pingos occur. Pingos are material uplifted by an ice core, creating symmetrical hills that may be up to 0.4 kilometers in diameter.