Not far beneath the surface of the Coral Sea, where the Great Barrier Reef lives, parrotfish teeth grind against rock, crab claws snap as they battle over hiding spots, and a 600-pound grouper pulses its swim bladder to announce its presence with a muscular whump. Sharks and silver jacks flash by. Anemone arms flutter and tiny fish and shrimp seem to dance a jig as they guard their nooks. Anything that can't glom on to something rigid is tugged and tossed by each ocean swell.
The reef's sheer diversity is part of what makes it great. It hosts 5,000 types of mollusks, 1,800 species of fish, 125 kinds of sharks, and innumerable miniature organisms. But the most riveting sight of all—and the main reason for World Heritage status—is the vast expanse of coral, from staghorn stalks and wave-smoothed plates to mitt-shaped boulders draped with nubby brown corals as leathery as saddles. Soft corals top hard ones, algae and sponges paint the rocks, and every crevice is a creature's home. The biology, like the reef, transforms from the north—where the reef began—to the south. The shifting menagerie is unmatched in the world.
Time and tides and a planet in eternal flux brought the Great Barrier Reef into being millions of years ago, wore it down, and grew it back—over and over again. Now all the factors that let the reef grow are changing at a rate the Earth has never before experienced. This time the reef may degrade below a crucial threshold from which it cannot bounce back.