Earth is the third planet from the Sun and the only known celestial object capable of harboring and supporting life. Land, which consists of continents and islands, covers 29.2 percent of the Earth’s surface. The remaining 70.8 percent is covered by water, largely in the form of oceans, seas, gulfs, and other salt-water bodies, but also in the form of lakes, rivers, and other freshwater bodies, which comprise the hydrosphere. The polar areas of the Earth are mostly covered with ice. The Earth’s outer layer is divided into several rigid tectonic plates that migrate across the surface over many millions of years, while its interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field, and a convective mantle that drives plate tectonics.