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.
West Meets Reef
Europeans were introduced to the Great Barrier Reef by British explorer Capt. James Cook, who came upon it quite by accident. On a June evening in 1770, Cook heard the screech of wood against stone; he couldn't have imagined that his ship had run into the most massive living structure on Earth: more than 10,000 square miles of coral ribbons and isles waxing and waning for some 1,400 winding miles.
Cook's team had been exploring the waters offshore of what is now Queensland when the H.M.S. Endeavour became trapped in the labyrinth. Not far beneath the surface, jagged towers of coral tore into the ship's hull and held the vessel fast. As timbers splintered and the sea poured in, the crew arrived on deck "with countenances which sufficiently expressed the horrors of our situation," Cook later wrote in his diary. Captain and crew were able to limp to a river mouth to patch their vessel.
Aborigines had lived in the region for thousands of years before Europeans hit the rocks. Culturally, the reef has been a rich part of the landscape for Aboriginal and Torres Strait Islander peoples, who have canoed it and fished it and shared myths about its creatures for generations. But historians aren't sure how deep their knowledge went of the reef's geology and animal life. A few decades after Cook's run-in with the behemoth beneath the sea, English cartographer Matthew Flinders—who also had a mishap or two while "threading the needle" among the reefs—gave the entity its name, inspired by its size. All told, if the reef's main chunks were plucked from the sea and laid out to dry, the rock could cover all of New Jersey, with coral to spare.
Expansion and Erosion
This mammoth reef owes its existence to organisms typically no bigger than a grain of rice. Coral polyps, the reef's building blocks, are tiny colonial animals that house symbiotic algae in their cells. As those algae photosynthesize—using light to create energy—each polyp is fueled to secrete a "house" of calcium carbonate, or limestone. As one house tops another, the colony expands like a city; other marine life quickly grabs on and spreads, helping cement all the pieces together.
Off Australia's eastern edge, conditions are ripe for this building of stone walls. Corals grow best in shallow, clear, turbulent water with lots of light to support photosynthesis. Millions of polyp generations later, the reef stands not as a singular thing but as a jumble whose shapes, sizes, and life-forms are determined by where in the ocean they lie—how close to shore, for example—and what forces work on them, such as heavy waves. Go far enough from the coast, where the light is low and the waters are deeper, and there's no reef at all.
"In the Great Barrier Reef, corals set the patterns of life from end to end," says Charlie Veron, coral expert and a longtime chief scientist for the Australian Institute of Marine Science. With over 400 species in the region, "they structure the entire environment; they're the habitat for everything else here." The perfect temperature, clarity, and currents enable plate corals, for example, to increase in diameter up to a foot a year. The reef continuously erodes as well, worn down by waves, ocean chemistry, and organisms that eat limestone. This vanishing act is far slower than the constant building up; still, as much as 90 percent of the rock eventually dissipates into the waters, forming sand. So the living veneer of this reef, the part a diver sees, is ever changing.
And the layers beneath are relatively young, geologically speaking, at less than 10,000 years. The reef's true beginnings go back much further. Closer to 25 million years ago, Veron says, as Queensland edged into tropical waters with the movement of the Indo-Australian tectonic plate, coral larvae began riding south-flowing currents from the Indo-Pacific, grabbing footholds wherever they could. Slowly, rocky colonies grew and spread along the seafloor flush with diverse marine life.