In the center of the Arctic is a large basin occupied by the Arctic Ocean. The basin is nearly surrounded by the ancient continental shields of North America, Europe, and Asia, with the geologically more recent lowland plains, low plateaus, and mountain chains between them. Surface features vary from low coastal plains (swampy in summer, especially at the mouths of such rivers as the Mackenzie, Lena, Yenisei, and Ob) to high ice plateaus and glaciated mountains. Tundras, extensive flat and poorly drained lowlands, dominate the regions. The most notable highlands are the Brooks Range of Alaska, the Innuitians of the Canadian Arctic Archipelago, the Urals, and mountains of E Russia. Greenland, the world's largest island, is a high plateau covered by a vast ice sheet except in the coastal regions; smaller ice caps are found on other Arctic islands. Because ice is a significant component of arctic lands and shores, sometimes alone and sometimes as a component of frozen soils, coastal erosion due to global warming is much greater in the Arctic than elsewhere.