Coral reefs are aragonite structures produced by living organisms, found in marine waters containing few nutrients. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate. The accumulation of skeletal material, broken and piled up by wave action and bioeroders, produces a calcareous formation that supports the living corals and a great variety of other animal and plant life.
Coral reefs most commonly live in tropical waters, but deep water and cold water corals exist on a much smaller scale.
Globally, coral reefs are under threat from climate change, ocean acidification, 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.