The Great Barrier Reef is the largest collection of coral reefs in the world, and is the biggest single structure made by living organisms in the world.
Hundreds of broken chains of coral reefs extend for about 1,250 miles (2,012 km) off the northeastern coast of Australia.
The Great Barrier Reef is home to hundreds of species of fish and other sea life, in addition to 350 species of living corals, the small plantlike animals that give the reefs their colorful appearance.
The limestone formations that shape the reefs are made up of the calcified skeletons of the coral organisms.
In recent years, the health of the Great Barrier Reef has been threatened by human actions, such as tourism, pollution, and oil exploration, as well as by natural menaces such as cyclones and invasions of crown of thorns starfish, which feed on living corals.