The Great Barrier Reef is the largest coral reef system in the world, composed of over 2,900 individual reefs and 900 islands stretching for 2,600 kilometres (1,600 mi) over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland in northeast Australia.

The Great Barrier Reef can be seen from outer space and is the world's biggest single structure made by living organisms. This reef structure is composed of and built by billions of tiny organisms, known as coral polyps. The Great Barrier Reef supports a wide diversity of life, and was selected as a World Heritage Site in 1981. CNN has labelled it one of the 7 natural wonders of the world. The Queensland National Trust has named it a state icon of Queensland.

A large part of the reef is protected by the Great Barrier Reef Marine Park, which helps to limit the impact of human use, such as overfishing and tourism. Other environmental pressures to the reef and its ecosystem include water quality from runoff, climate change accompanied by mass coral bleaching, and cyclic outbreaks of the crown-of-thorns starfish.

The Great Barrier Reef has long been known to and utilised by the Aboriginal Australian and Torres Strait Islander peoples, and is an important part of local groups' cultures and spirituality. The reef is a very popular destination for tourists, especially in the Whitsundays and Cairns regions. Tourism is also an important economic activity for the region. Fishing also occurs in the region, generating AU$ 1 billion per year.

Physiography

The Great Barrier Reef is a distinct physiographic province of the larger East Australian Cordillera division. It encompasses the smaller Murray Islands physiographic section.

Geology and geography

The Great Barrier Reef reaches from Torres Strait (between Bramble Cay, its northernmost island, and the south coast of Papua New Guinea) in the north to the unnamed passage between Lady Elliot Island (its southernmost island) and Fraser Island in the south. Lady Elliot Island is located southeast of Bramble Cay as the crow flies.

Australia has moved northwards at a rate of per year, starting during the Cainozoic. Eastern Australia experienced a period of tectonic uplift, leading to the drainage divide in Queensland moving inland. Also during this time, Queensland experienced volcanic eruptions leading to central and shield volcanoes and basalt flows. Some of these granitic outcrops have become high islands. After the Coral Sea Basin was formed, coral reefs began to grow in the Basin, but until about 25 million years ago, northern Queensland was still in temperate waters south of the tropics - too cool to support coral growth. The history of the development of the Great Barrier Reef is complex; after Queensland drifted into tropical waters, the history is largely influenced by how reefs fluctuate (grow and recede) as the sea level changes. They can increase in diameter from per year, and grow vertically anywhere from 1 to 25 centimetres (0.4–12 in) per year; however, they are limited to growing above a depth of due to their need for sunlight, and cannot grow above sea level. When Queensland moved into tropical waters 24 million years ago, some coral grew, but a sedimentation regime quickly developed with erosion of the Great Dividing Range; creating river deltas, oozes and turbidites, which would have been unsuitable conditions for coral growth. 10 million years ago, the sea level significantly lowered, which further enabled the sedimentation. The substrate of the GBR may have needed to build up from the sediment until the edge of the substrate was too far away for suspended sediments to have an inhibiting effect on coral growth. In addition, approximately 400,000 years ago there was a particularly warm interglacial period with higher sea levels and a 4 degree Celsius (7.2 degree Fahrenheit) change in water temperature.