Antarctica is the fifth largest continent and is asymmetrically centered on the South Pole and almost entirely within the Antarctic Circle. Antarctica consists of two major regions: W Antarctica (c.2,500,000 sq mi/6,475,000 sq km), a mountainous archipelago that includes the Antarctic Peninsula, and E Antarctica (c.3,000,000 sq mi/7,770,000 sq km), geologically a continental shield. They are joined into a single continental mass by an ice sheet thousands of feet thick. At the seaward margins of the ice sheet masses of ice break off and float away as icebergs, leaving ice cliffs. Where the outward creep of the ice is channeled into ice streams (zones of more rapid flow), great floating ice tongues project into the sea; where mountains retard outward movement, the flow is channeled into great valley glaciers. Less than 5% of Antarctica is free of ice; these areas include mountain peaks, arid dry valleys, small coastal areas, and islands. There is no native human population in Antarctica, nor are there any large land animals. Few species are adapted to the antarctic environment, but individuals of these few species are numberless.