This year saw the worst mass die-off of coral ever recorded at the Great Barrier Reef, with the "most-pristine" parts of the reef being hit the hardest, the Guardian reports.

One 430-mile section in the northern portion of the 1,400-mile-long reef lost 67 percent of its shallow-water coral. All told, the Great Barrier Reef lost 22 percent of its coral in 2016's bleaching event.

"To see those sections, two-thirds of the northern section, dead, is catastrophic," an Australian environmental minister tells the New York Times. The northern section of the reef had historically been the least damaged by human activity.

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The coral bleaching was caused by warming waters; the southern portion of the reef escaped great harm thanks to a cyclone that lowered water temperatures. This was the third—and by far the worst—coral bleaching event to hit the Great Barrier Reef in the past 18 years, the AP reports.

Since the last event in 2002, the reef's coral had increased by 19 percent, according to the Australian government. Experts say it could take 10 to 15 years to regain the coral lost in this year's bleaching event—but that's assuming there isn't a fourth during that span.

One expert says that even if the Great Barrier Reef hits pre-bleaching levels of coral, it will never regain the diversity it once had. The UN is considering adding the Great Barrier Reef to its list of world heritage sites that are in danger.

And the Australian government plans to spent $1.5 billion on protecting the reef over the next decade. (A huge reef was discovered hiding behind the Great Barrier Reef.)

This article originally appeared on Newser: Great Barrier Reef Lost 22% of Its Coral This Year