Seven Natural wonders of the world

Great Barrier Reef

The Great Barrier Reef in Australia is the world's largest coral reef system, composed of roughly 3,000 individual reefs and 900 islands that stretch for 2,600 kilometres (1,616 mi) and cover an area of approximately 344,400 km². The reef is located in the Coral Sea, off the coast of Queensland in northeast Australia. A large part of the reef is protected by the Great Barrier Reef Marine Park.
The Great Barrier Reef can be seen from outer space and is sometimes referred to as the single largest organism in the world. In reality, it is made up of many millions 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 seven natural wonders of the world. The Queensland National Trust has named it a state icon of Queensland.[6]
The Great Barrier Reef's environmental pressures include water quality from runoff, climate change and mass coral bleaching, cyclic outbreaks of the crown-of-thorns starfish, overfishing, and shipping accidents.

Geology and geography
The Reef Research Centre, a Cooperative Research Centre, has found coral 'skeleton' deposits that date back half a million years. Corals have been growing in the region for as long as 25 million years, but have not always formed coral reefs.
Dating discrepancies stem from how reefs fluctuate (grow and recede) as the sea level changes. They can increase in diameter from 1 to 2 centimetres per year, and grow vertically anywhere from 1 to 15 centimetres per year; however, they are limited to growing above a depth of 150 metres due to their need for sunlight, and cannot grow above sea level.
According to the Great Barrier Reef Marine Park Authority, the current, living reef structure is believed to have begun growing on an older platform about 20,000 years ago.[8] The Australian Institute of Marine Science agrees, which places the beginning of the growth of the current reef at the time of the Last Glacial Maximum. At around that time, the sea level was 120 metres lower than it is today. The land that formed the substrate of the Great Barrier Reef was a coastal plain with some larger hills (some of which were themselves remnants of older reefs).
Heron Island, a coral cay in the southern Great Barrier Reef.