CANBERRA, Australia—An underwater heat wave is devastating huge swaths of Australia’s Great Barrier Reef, marine researchers have found.

An aerial survey of the chain of 3,000 coral outcrops—a Unesco World Heritage site and the only living system visible from space—found 95% of its northern area, roughly half of the reef’s length, had been hit by a bleaching event that began six months ago. Damage to the southern area is still being assessed.

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