Arctic landscapes are other worldly. Great glaciers coat the northern mountains with a white frosting that can be a kilometre thick. At other locations, the land lies exposed. In many places, these polar landscapes provide perfect classrooms for geology – bedrock lies on the surface or jumbles of boulders signal the retreat of glaciers. In other areas, the ground is covered with finer particles, reflecting the presence of rocks that were ground to powder by glaciers or were sediments uplifted from the sea. Viewed from the air, these surfaces show patterns of cracks and mounds that make the landscape look like the hide of some primeval beast. In a few regions, ice volcanoes erupt from the tundra.