How was the Great Barrier Reef formed?

The Great Barrier Reef is a vast complex of coral reefs, shoals, and islets off the northeastern coast of Australia. It was formed over millions of years from the accumulated skeletons and skeletal waste of marine organisms, primarily coral polyps and coralline algae. These tiny creatures secrete calcium carbonate, which forms the rigid framework of the reef. Over time, this framework is cemented together and filled in by skeletal debris, creating the massive structure we see today. The reef has grown on the shallow continental shelf in warm waters, with evidence of reef growth dating back to the Miocene Epoch. Changes in sea level over the last two million years have also influenced its development.123