The Biosphere: Realm of Life

Earth is home to more than 30 million different kinds of living things, some of which are still being discovered. Life ranges in size from the giant redwood tree to tiny bacteria so small that 25,000 of them lined up would only stretch 1 inch. Life, large or small, covers almost every square inch of Earth's surface - this is the biosphere, extending downward about five miles to the deep ocean and upward about 5 miles to the mountaintops. It is the most dynamic and fragile realm in a complex system of interacting spheres that make up the planet.