Coral reefs are underwater structures made from calcium carbonate secreted by corals. Corals are colonies of tiny living animals found in marine waters containing few nutrients. Most coral reefs are built from stony corals, and are formed by polyps that live together in groups. The polyps secrete a hard carbonate exoskeleton which provides support and protection for the body of each polyp. Reefs grow best in warm, shallow, clear, sunny and agitated waters.

Often called “rainforests of the sea”, coral reefs form some of the most diverse ecosystems on earth. They occupy less than one tenth of one percent of the world ocean surface, about half the area of France, yet they provide a home for twenty-five percent of all marine species, including fish, molluscs, echinoderms and sponges.

Paradoxically, coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, particularly in the Pacific Ocean, but deep water and cold water corals also exist on smaller scales.

Coral reefs deliver ecosystem services to tourism, fisheries and shoreline protection. The annual global economic value of coral reefs has been estimated at $30 billion. However, coral reefs are fragile ecosystems, partly because they are very sensitive to water temperature. They are under threat from climate change, ocean acidification, blast fishing, cyanide fishing for aquarium fish, overuse of reef resources, and harmful land-use practices. High nutrient levels such as those found in runoff from agricultural areas can harm reefs by encouraging excess algae growth.

Most coral reefs were formed after the last glacial period when melting ice caused the sea level to rise and flood the continental shelves. This means that most coral reefs are less than 10,000 years old. As coral reef communities were established on the shelves, they built reefs that grew upwards, keeping pace with the rise in sea level. Reefs that didn't keep pace could become drowned reefs, covered by so much water that there was insufficient light for further survival.

Coral reefs are also found in the deep sea away from the continental shelves, around oceanic islands and as atolls. The vast majority of these ocean coral islands are volcanic in origin. The few exceptions have tectonic origins where plate movements have lifted the deep ocean floor on the surface.

In 1842 Charles Darwin published his first monograph, The Structure and Distribution of Coral Reefs. There he set out his theory of the formation of atoll reefs, an idea he conceived during the voyage of the Beagle. His theory was that atolls were formed by the uplift and subsidence of the Earth's crust under the oceans. Darwin’s theory sets out a sequence of three stages in atoll formation. It starts with a fringing reef forming around an extinct volcanic island as the island and ocean floor subsides. As the subsidence continues, the fringing reef becomes a barrier reef, and ultimately an atoll reef.

Darwin’s theory starts with a volcanic island which becomes extinct

As the island and ocean floor subside, coral growth builds a fringing reef, often including a shallow lagoon between the land and the main reef

As the subsidence continues the fringing reef becomes a larger barrier reef further from the shore with a bigger and deeper lagoon inside