Triggered by heat stress in warm oceans, coral bleaching occurs when corals expel the colourful algae living in their tissues. Without these helpful algae, the corals become pale

STORY: Once vibrant in color... much of the world's coral reefs have turned completely white. "This is the biggest coral bleaching event on record."In an exclusive interview with Reuters, Derek Manzello of the National Oceanic and Atmospheric Administration said about 77% of the world's coral reefs have been subject to bleaching-level heat stress - the most extensive on record.