Stretching more than 2,300 kilometres along the coast of Queensland, the Great Barrier Reef is the largest coral reef system on Earth. It is not one reef but a chain of around 2,900 separate reefs and 900 islands, covering an area larger than Victoria and Tasmania combined. It is so big that it can be seen from space.

What is coral?

Many people think coral is a plant or a rock, but it is actually an animal. A piece of coral is made up of thousands of tiny creatures called polyps. Each polyp builds a hard skeleton of limestone around itself, and over hundreds of years these skeletons pile up to form a reef. Living inside the polyps are microscopic algae. The algae use sunlight to make food, which they share with the coral. In return, the coral gives the algae a safe place to live. The algae also give coral its bright colours.

A city under the sea

The reef is home to more than 1,500 species of fish, around 400 types of coral, and six of the world's seven species of sea turtle. Dugongs graze on seagrass meadows, humpback whales visit each winter to give birth, and giant clams more than a metre wide sit wedged between the corals. Aboriginal and Torres Strait Islander peoples have lived alongside the reef for tens of thousands of years, and more than 70 Traditional Owner groups continue to care for their sea country today.

A reef under pressure

In 1981, the reef was added to the World Heritage List to protect it for future generations. But the reef faces serious threats. When ocean water becomes too warm, stressed corals push out the algae living inside them. Without the algae, the coral turns ghostly white. This is called coral bleaching. Bleached coral is not dead, and it can recover if the water cools quickly. If the heat lasts too long, however, the coral starves.

Other dangers include soil and fertiliser washing into the sea from farms, which clouds the water and blocks sunlight, and outbreaks of the crown-of-thorns starfish, which eats coral.

Working together

Scientists, rangers, farmers and Traditional Owners are working together to give the reef the best chance of survival. Divers remove crown-of-thorns starfish by hand, farmers are changing the way they use fertiliser, and researchers are growing heat-tolerant corals in nurseries. The reef has survived for thousands of years. Whether it survives the next hundred depends on what people do now.