The Great Barrier Reef is the largest structure on Earth built by living things: about 2,900 separate reefs running 2,300 kilometres down the coast of Queensland, Australia, and the point of coming is to put a mask on and float over it. From Cairns the boats go out past the inshore reefs to the outer wall, where the coral grows in terraces of purple, mustard and rust and clouds of damselfish and parrotfish rearrange themselves beneath you. Around 1,500 species of fish live here, along with turtles grazing the seagrass and reef sharks passing below the drop-off. Further north, on the Ribbon Reefs, is Cod Hole, where potato cod the size of a person swim straight up to divers and hang at eye level, unbothered. In June and July the same reefs draw dwarf minke whales, and this is the only place in the world where they turn up reliably enough for boats to go out and swim with them.
The reef is also seen from above. Flights over the Whitsundays pass Heart Reef, a small heart-shaped coral set in the maze of Hardy Reef, and the white swirls of sand at Whitehaven Beach. Some of the reef's best moments belong to particular nights. A few nights after the November full moon the corals off Cairns and Port Douglas all spawn at once, releasing eggs and sperm that drift upward like a snowstorm in reverse. On the southern coral cays, Lady Elliot and Heron among them, green and loggerhead turtles come ashore to nest from November to March, and the hatchlings dig their way out from January. The living coral you see is young; it grows on the remains of reefs that formed and died over hundreds of thousands of years as the sea rose and fell. On the night of 11 June 1770 James Cook's ship struck this coral and nearly sank, which is how the outside world first learned it was here; for the Aboriginal and Torres Strait Islander peoples of the coast it has always been Sea Country, part of their law and their stories.