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From space, the east coast of Australia appears to be in the embrace of a giant opal. The largest living structure on Earth, the Great Barrier Reef, is a lacy, living wall...spanning more than 2,000 kilometers of islands and submerged reefs,,,between the Queensland coast and the western edge of the Pacific Ocean. Diving in, the opal seems to splinter into millions of pieces. Whirlpools of small, metallic blue fish; barracuda, gliding like silver submarines; occasionally, a lone predatory shark. The Great Barrier Reef is like an underwater city whose buildings are alive with millions of small creatures,,,whose lives are intimately and intricately connected. It is as diverse as a rainforest – a mosaic of more than 70 types of habitats hosting thousands of species of marine life. As many as a hundred different kinds of coral may occupy a single acre of ocean. Molecule by molecule, coral animals gradually extract calcium carbonate from the surrounding water...to form minute stony cups around each animal’s soft crown of tentacles. Some corals live in solitary splendor, but most are built with hundreds, sometimes thousands...of individual animals linked together to form a single coral mound, plate, or cluster of branches. Some are like little trees and shrubs. They provide food and shelter for thousands of other forms of life. Corals get the credit for most of the reef structure, but much of the construction is done by fast-growing, encrusting red algae. They act like pink glue, cementing fragments of shells, sand, and coral with sheets of calcium carbonate.The reef is home to more than 4,000 kinds of mollusks. From tiny sea slugs, nudibranchs, to giant clams. Green sea turtles travel thousands of miles in the open sea...to reach the sandy beaches of some of the Barrier Reef’s islands, and there, to lay their eggs. Hatchlings head straight for the sea. They will travel thousands of miles over the years and eventually return to lay their own eggs. Established as a national park in 1975, the Great Barrier Reef was designated as a World Heritage site 6 years later. Today, about 33% of it is fully protected from fishing and other extractive activities...and efforts are underway to deal with pollution, overfishing, and the consequences of climate change. The Great Barrier Reef appears to be about 20,000 years old. But geologists using deep quarrying techniques have found evidence of ancient corals there that are half a million years old. With care, the future of Australia’s living treasure – the Great Barrier Reef – will be at least as enduring as its magnificent past.