Coral reef

A living limestone structure built by coral polyps and algae, hosting about a quarter of all marine species and most tropical dive sites.

A coral reef is a living structure built over centuries by colonies of coral polyps, tiny animals that secrete calcium carbonate skeletons and host symbiotic algae called zooxanthellae, which give corals their color and most of their energy. Reefs need warm, clear, sunlit water, which is why the classic reef belt spans the tropics: the Red Sea, the Coral Triangle of Indonesia and the Philippines, the Caribbean, and Australia's Great Barrier Reef, the largest living structure on Earth at over 2,000 km long.

Reefs concentrate life like nothing else in the ocean, supporting roughly a quarter of known marine species on a fraction of a percent of the seafloor. For divers they offer structure at every depth: reef flats and lagoons for training dives, sloping gardens, and walls that drop into the blue. Fringing reefs hug the shore, barrier reefs stand offshore behind a lagoon, and atolls ring a vanished volcanic island.

Reef etiquette is core dive skill: perfect buoyancy control, no touching, and reef-safe sunscreen, because bleaching driven by warming water has already damaged large areas. A logbook that records which reefs you have seen, and in what condition, becomes a small personal record of a changing ocean. Even a simple healthy or bleached note per dive adds up over the years.