pingo

 A mound of earth-covered ice that forms in permafrost regions when water is forced upward through the frozen ground and freezes, pushing the surface into a dome. Pingos can be 200 feet tall and 2,000 feet in diameter. They are among the most striking landforms of the Arctic — solitary, symmetrical hills rising from an otherwise flat tundra. When a pingo collapses, it leaves a circular depression that may fill with water.