Which reef system is the world's largest structure built by living organisms?

The Great Barrier Reef is the world's largest structure built by living organisms.

It lies off the northeastern coast of Australia and extends for about 2,300 kilometres. The system contains thousands of individual reefs and hundreds of islands, formed mainly from the calcium-carbonate skeletons of reef-building corals. It supports an exceptionally diverse community of fish, molluscs, echinoderms, turtles, seabirds, and marine mammals.

The Great Barrier Reef is not one continuous wall of coral. It is a vast network shaped by sea-level changes, coral growth, currents, storms, and geological history. Its living corals are vulnerable to heat stress, which can cause bleaching when corals lose their symbiotic algae.

Australia established the Great Barrier Reef Marine Park in 1975, and UNESCO listed the area as a World Heritage Site in 1981. The reef’s enormous size does not make every part equally healthy; conditions vary among reefs and across years.