View towards southeast. The Great Barrier Reef is the most extensive coral reef system worldwide, with almost 3000 individual reefs on a length of 2300 kilometres and a surface area of almost 350000 square kilometres. It was declared a UNESCO World Heritage in 1981. This image shows the Hook Reef in the foreground and the Hardy Reef in the background, separated by a deep tidal channel. The permanent structures in the left portion of the image facilitate visitors to closely observe the reef ecosystem.