Scientists surveying the northern Great Barrier Reef say they've witnessed the most severe coral bleaching on record.
The phenomenon is caused by abnormally high sea temperatures, which drains corals of their colour and can kill them.
Out of the 520 coral reefs surveyed in the most pristine section of the Great Barrier Reef, scientists only found four that had no bleaching.
Bleached coral can be saved if temperatures cool, but scientists have been reporting a mortality rate of up to 50 percent.
Professor Terry Hughes said it was the saddest research trip of his life.
"This is an ongoing, slow-motion train wreck," he told AAP.