Two-thirds of the Earth’s surface is covered by seawater, which makes up 97 percent of the planet’s entire water supply. The largest expanses of water are called oceans, while smaller ones (usually close to or partially enclosed by land) are called seas.
The ocean floor is made up of mountains, valleys, plateaus, and plains as complex as geological formations on land. The area of the ocean floor along coastlines is called the continental shelf, where the water is relatively shallow. At the outer edge of the continental shelf, the ocean floor drops away steeply.
Oceans are home to some of the most diverse life on Earth. Plants live only at the upper zone of the ocean (the sunlit zone), where there is enough light for growth. Animals, however, are found at all depths of the ocean, and many are practically unknown to humans because of the technological challenges associated with going deep enough to encounter some of the bottom dwellers. Some animals live in several zones of the ocean; the sperm whale, for example, dives very deep to feed but returns to the surface to breathe air.
Kelp is perhaps the best-known ocean plant. Each leaflike blade of a kelp plant has an air bladder that keeps the plant afloat so that it can maximize its absorption of sunshine, necessary for staying alive and growing.