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The word ocean refers to one of the Earth's four distinct, large areas of salt water, the Pacific, Atlantic, Indian, and Arctic Oceans. The word can also mean the entire network of water that covers almost three quarters of our planet. It comes from the Greek Okeanos, a river believed to circle the globe. The word sea can also mean the vast ocean covering most of the world. But it more commonly refers to large landlocked or almost landlocked salty waters smaller than the great oceans, such as the Mediterranean Sea or the Bering Sea. Sailors have long referred to all the world's waters as the seven seas. Although the origin of this phrase is not known for certain, many people believe it referred to the Red Sea, the Mediterranean Sea, the Persian Gulf, the Black Sea, the Adriatic Sea, the Caspian Sea, and the Indian Ocean, which were the waters of primary interest to Europeans before Columbus.