The Great Barrier Reef is in the throes of its worst-ever coral bleaching event, scientists say. Only 7% of the 2,300km-long reef system has been left untouched by bleaching, with researchers expressing alarm over the health of one of the world's greatest natural wonders. Climate change and an unusually strong El Nino have been blamed for the latest mass bleaching event, which has resulted in vast swathes of coral reefs turning white. Bleached corals can die if they are under prolonged stress.