Snow is a good insulator, Schweiger explains, so the ground beneath the snow is significantly warmer than the air above it. As the snow builds up, the ground gives off heat and vapourizes the snow next to it. The water vapour condenses and freezes on the snow above it, creating an open space at ground level. The temperature next to the ground is around zero degrees Celsius through the winter. The inhabitants of the subnivean space are the smallest forest mammals, like mice, voles, and shrews.
Some predators live there too, particularly weasels.
A whole food chain develops under the snow over the winter. You can often see evidence of the life under the snow by observing the edges of ski or snowmobile trails. The trails compress the snow and break through the subnivean space, often cutting across natural subnivean tracks or runways. Where this has happened, you will see tunnels emerging from either side of the trail and, sometimes, a scurry of small footprints crossing the trail from one tunnel to the other. Predators above the snow also take advantage of the residents of the subnivean space.