Nothing alive has ever built anything bigger than the Great Barrier Reef: about 2,900 separate reefs running 2,300 kilometres down the Queensland coast of Australia. It is a thing you get inside rather than look at. Boats leave Cairns and run past the silty inshore reefs to the outer wall, where the water turns blue and the coral sits shallow enough for a snorkel. You drift face down over terraces of purple, mustard and rust while clouds of damselfish and parrotfish rearrange themselves underneath you, a few of the 1,500 fish species here. Turtles graze the seagrass, and reef sharks cruise below the drop-off. Further north, on the Ribbon Reefs, potato cod as long as a person come straight up to divers at Cod Hole and hang at eye level, entirely unbothered. In June and July the same reefs draw dwarf minke whales, the only place on Earth they gather this reliably; you hold a rope at the surface and wait, because the whales decide.
Part of it can only be seen from the air. Light aircraft cross the Whitsundays over Hardy Reef, where a small heart-shaped patch of coral sits in the maze, closed to swimmers, divers and boats alike because it is too fragile to take anyone. The same flights usually pass the swirling white sand of Whitehaven Beach. What you float over is younger than it looks. Every ice age drops the sea and kills the reef, every warm spell lets it grow back off the drowned limestone underneath, and those foundations may go back half a million years. That cycle has not stopped. Late summer is when the reef bleaches, and water that stays too warm leaves whole stretches pale. Then, two to six nights after November's full moon, the corals off Cairns and Port Douglas all spawn on the same night, and the dark water fills with tiny bundles of eggs and sperm going up instead of down. Doing it together swamps the predators that would eat them. For those few nights the reef is building the part of itself you will float over next.