Life on Earth began in the ocean and thrives there in staggering diversity. Creatures great and small are intertwined in the life cycle of the saltwater seas. Tiny microscopic plankton are just as important to its balance as the largest animal ever to live on Earth—the blue whale—which at almost 150 tons is the size of a jet plane!
Oceans cover almost 75 percent of the world's surface and give Earth its blue appearance when seen from outer space. This abundance of water is unique in our solar system. While not Earth's only source of water, oceans contain 97 percent of all the water on the planet.
Throughout tropical oceans, colorful coral reefs cover about 100,000 square miles (260,000 square kilometers) and are estimated to host half a million different species. These reefs teem with marine animals, which find shelter in their nooks and crannies and thrive in the shallow, warm waters where coral grows. Coral reefs are fragile and grow slowly. A hundred years may pass before an inch (2.5 centimeters) of growth!
Where oceans meet land, tides wash in twice a day, so most wildlife that lives in coastal habitats copes well with change. Animals on rocky shores, such as barnacles and periwinkle snails, may avoid being swept out to sea by clinging to rocks. Others, such as sand crabs and mussels, burrow into sand or mud.
Though people know oceans by many different names, there's actually just one ocean, because Earth is covered by a single layer of connecting water. The ocean is vital to people and wildlife alike, but only people have the power to protect Earth's largest habitat.
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Blue Frontier: The Sustainable Seas Expeditions Explore America's Wild Blue Under—the U.S. National Marine Sanctuaries.
International Year of the Ocean The ocean is so vital to the Earth's health that it had its own year—1998. Relive it here, get fact sheets and activities, and more.
SeaWorld/Busch Gardens
SeaWorld/Busch Gardens has teacher guides and animal resources online, as well as related sites.
Treasures@SeaExplore the ocean with interactive, writing, book, and art activities.
U.S. National Marine Sanctuaries See photo galleries, get sanctuary news, and more.
U.S. National Oceanic and Atmospheric Administration NOAA scientists study the oceans, our atmosphere, outer space, and the sun. See what they've learned.
