In past bleaching events on the Great Barrier Reef, the southern region has sometimes been spared worst of the bleaching. Not this time. This year's intense underwater heat has triggered the most severe heat stress ever seen on record. Only 3% of surveyed southern reefs have not bleached at all. It's shaping up to be the most severe and widespread bleaching of the southern reef, while mass bleaching has hit other areas of the reef in the fifth mass bleaching event in eight years.