Coral reefs are scattered throughout the tropical and subtropical western Atlantic and Indo-Pacific oceans. About 600,000 km² (231,600 mi²) of coral reefs are known to exist, comprising 0.17% of the ocean's surface. The Great Barrier Reef off northeastern Australia in the Indo-Pacific is the largest barrier reef in the world, stretching more than 2,000 km (1,240 miles).

Reefs are built by vast colonies of coral polyps - tiny invertebrate animals related to sea anemones and jellyfishes. Reef-building corals secrete calcium carbonate skeletons. The calcium carbonate gradually accumulates and the reef becomes large.

Coral reefs are one of the most biologically diverse ecosystems, rivaling rain forests for the number of species inhabiting them. About 6,000 to 8,000 fish species as well as sponges, sea anemones, bryozoans, worms, sea stars, crustaceans, and snails live on coral reefs.

In addition to the array of marine life they support, reefs also protect coastlines from erosion, provide food and recreation for people, and are proving to be a new frontier for medical research. Unfortunately, reefs are threatened worldwide by pollution, development, and coral harvesting. More than one-third of all coral reefs have already been destroyed or permanently damaged.

To strengthen efforts for coral reef management and conservation, the U.S. Departments of State and Commerce joined with nations around the globe and declared 1997 the International Year of the Reef. You can help protect reefs by not purchasing coral or other living souvenirs and by keeping the ocean clean. If you plan to visit a coral reef, learn how to safely enjoy this living system.

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