Great Barrier Reef: Diving the World's Largest Living Structure

The Great Barrier Reef is the largest coral reef system on Earth, comprising over 2,900 individual reefs and 900 islands stretching across 344,400 square kilometres of the Coral Sea off the coast of Queensland, Australia. It is the world's biggest single structure made by living organisms — the coral polyps that build the reef are tiny animals, each no larger than a fingernail, but over thousands of years their accumulated calcium carbonate skeletons have created a structure so large it is visible from space. The reef was designated a UNESCO World Heritage Site in 1981 and is one of the seven natural wonders of the world.

The biodiversity of the Great Barrier Reef is staggering — the reef supports over 1,500 species of fish, 4,000 species of mollusc, 240 species of bird, and six of the world's seven species of marine turtle. The coral itself comprises over 400 species, ranging from the massive brain corals and table corals of the outer reef to the delicate branching staghorn corals of the shallower lagoons. The reef is also home to dugongs — large, gentle herbivores related to manatees — and the world's largest population of humpback whales passes through its waters on their annual migration.

The best diving on the Great Barrier Reef is found on the outer reef, where the coral walls drop hundreds of metres into the deep blue of the Coral Sea and the visibility can exceed 30 metres. The Cod Hole, near Lizard Island in the far north, is famous for its population of potato cod — enormous fish that can weigh up to 110 kilograms and are so accustomed to divers that they will approach within touching distance. The Ribbon Reefs, a series of narrow reefs running parallel to the coast north of Cairns, offer some of the finest wall diving in the world, with coral formations of extraordinary colour and complexity.

The reef faces severe threats from climate change, which has caused four mass bleaching events since 1998 — the most recent and most severe in 2022, which affected 91 percent of the reefs surveyed. Coral bleaching occurs when water temperatures rise above the coral's tolerance threshold, causing the coral to expel the symbiotic algae that give it its colour and provide up to 90 percent of its energy. Bleached coral is not dead, but if the stress continues for more than a few weeks, the coral starves and dies. The 2022 bleaching event was the first to occur during a La Niña year, when ocean temperatures are typically cooler — a sign of how dramatically the baseline temperature of the Coral Sea has risen.

Cairns, on the Queensland coast, is the main gateway to the Great Barrier Reef and offers a wide range of reef experiences, from day trips on large catamarans to liveaboard dive expeditions lasting several days. The Cairns Aquarium, opened in 2017, provides an excellent introduction to the reef's biodiversity for those who cannot dive or snorkel. The Daintree Rainforest, 100 kilometres north of Cairns, is the world's oldest tropical rainforest and meets the reef at Cape Tribulation — the only place on Earth where two UNESCO World Heritage Sites share a border.

The Whitsunday Islands, a group of 74 islands in the middle of the Great Barrier Reef Marine Park, offer a different experience of the reef — sailing between islands of tropical forest, anchoring in sheltered bays, and snorkelling on the fringing reefs that surround each island. Whitehaven Beach, on Whitsunday Island, is consistently rated one of the finest beaches in the world — a seven-kilometre stretch of silica sand so pure that it squeaks underfoot and does not retain heat. The Hill Inlet lookout, above the northern end of the beach, offers a view of the swirling patterns of sand and turquoise water that is one of the most photographed landscapes in Australia.

The best time to visit the Great Barrier Reef is from June to October, during the dry season, when the water is clearest and the weather is most reliable. The wet season from November to April brings heavy rain, strong winds, and the risk of cyclones, but also warmer water temperatures that can trigger bleaching. The stinger season — when box jellyfish and Irukandji jellyfish are present in the inshore waters — runs from October to May, and wearing a full-body lycra suit is recommended for swimming and snorkelling during this period. The outer reef is generally stinger-free year-round.