The Great Barrier Reef

THE GREAT BARRIER REEF (off the coast of Queensland, Australia)

The world's largest coral reef system, off the coast of Queensland, Australia, is 1,400 miles long and 133,000 square miles in area, with over 2,900 individual reefs and 900 islands. Built by billions of tiny organisms, it is the largest living structure on earth, and can be seen from outer space. The reef has lost more than half its coral cover since 1985.

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(Image courtesy Wikimedia Commons; CTTO)