Up until around 53 million years ago Australia was part of a vast super-continent, consisting of what is now South America, Antarctica and Australia, called Gondwanaland. At about this time Gondwanaland began to divide, allowing the Australian continent to slowly drift north at a few centimetres a year. The northern tip of Australia entered the tropics around 17 million years ago, meaning that for the first time Australia had a tropical coastline, with water temperatures conducive to a new range of life, which had previously been absent, including tropical reef-building corals. In addition, many species previously inhabiting the waters of the continental shelf had evolved into new tropical species.
The most significant period of coral growth and diversity began about 2 million years ago. Since then glacial periods lasting thousands of years have lowered the sea level revealing the continental shelf many times. Each time this occurs the reef is exposed and dies, leaving behind hills of limestone where living reef once flourished. When the glacial periods finish the sea level once again rises, slowly inundating the continental shelf. As this happens corals begin to re-establish and form new reefs with the cycle of life beginning anew.
The end result defies human description.