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Coral reefs are massive and complex structures made of limestone that is deposited by living sea organisms. The reefs are mainly composed of the skeletons of tiny, fragile animals called coral. Although there are hundreds of different species of corals, they are generally classified as either “hard coral” or “soft coral.” Hard corals grow in colonies. Their skeletons are made out of calcium carbonate, which hardens and eventually becomes rock (i.e., coral reefs). Soft corals are nonreef building corals often resembling plants or trees.

Coral reefs are found in over 100 countries and cover an estimated total area of 109,700 square miles (284,300 square kilometers) worldwide. Most reefs are located in oceans between the Tropics of Cancer and Capricorn, but they are also found farther from ...

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