Coral reefs are the largest structures of biological origin on Earth. These limestone formations were built up over thousands of years by colonies of small marine organisms called corals, and they are typically found in shallow warm-water seas off the continental shelf or fringing an island. As an individual coral -- known as a polyp -- grows, it secretes a substance that hardens to form calcium carbonate, or limestone, which provides the animal with a protective external skeleton. Most coral polyps manufacture this substance from a combination of waste products given off by tiny algae living in the coral's tissues and calcium ions absorbed from seawater. When a polyp dies, its skeletal remains become part of the foundation of the growing reef.

Coral reefs rival old-growth forests in terms of the diverse ecological communities they sustain and protect. Fishes of many kinds find protection and food in and around reef tops, while other marine animals, such as crabs, anemones, and eels, find shelter farther down under ledges and inside crevices.

With few exceptions, coral reefs tend to form in tropical and semitropical seas -- between 30° north and 30° south latitudes -- which support the production of large amounts of precipitated calcium carbonate. Most coral reefs require very salty water that is clear enough to permit high light penetration. This last requirement also explains why reefs are found only in shallow waters of about 70 meters (230 feet) or less, the distance that light can penetrate through water.

Coral reefs are sensitive, albeit resilient, ecosystems. One hundred years ago, virtually all of the world's coral reefs were healthy. Even when they sustained damage from destructive natural events such as hurricanes, freshwater inundation, and seismic activity, the reef ecosystems typically recovered. However, experts predict that current environmental pressures, including global climate change, emerging diseases such as bleaching, and the release of harmful sediments and chemicals from land-based agriculture and coastal development may kill more than half of the world's coral reefs by 2050.

Beyond their value to marine life, reefs also provide millions of people with food, income from fishing and tourism, coastal protection against waves and floods, and medications. Sustainable reef management programs, which involve ongoing health assessment, investigations into the major causes of reef destruction, eradication of non-native species, public awareness campaigns, and enforcement, can help minimize threats to the well-being of the reefs, especially damage caused by human activities.

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