Permafrost soils (soil that remains frozen for at least two years) of the Arctic tundra are covered with an "active layer" - a thin layer of soil that thaws when the summer sun warms the tundra surface.

Sometimes ice wedges form "polygons" - a honeycomb of ice walls beneath the soil surface. These can be about 70 feet / 20 meters across, although polygons may be as small as 10 feet / 3 meters across.

How does this happen?

• In the winter the cold causes the frozen soil to shrink, and cracks form.
• In the spring, the active layer melts and water seeps down into the cracks. It freezes and expands when it is chilled by the still-frozen soil. The frozen water forms wedges of ice in the soil.
• The active layer and the tops of the ice wedges melt in the summer, adding more water.
• Each winter, cracks form again in the same places, and in the spring, more water enters and enlarges the ice wedges as the freezing water expands.
• This cycle of crack, melt, and freeze enlarges the wedges year by year until the soil above them is pushed up, forming a blocky pattern of ridges on the ground called polygons.