Great Barrier Reef, Australia Composed of more than 3,000 individual reefs interspersed with more than 600 topical islands, the world's most extensive coral reef system is so large it can be identified from space. While the region has been protected since 1981, a 2012 study found that the delicate ecosystem has lost more than half of its coral since 1985 due to a combination of factors including coral bleaching caused by climate change.