In July 2023, Katey Lesneski dove into the ocean off Key Largo to visit Horseshoe Reef, one of the largest and healthiest natural stands of elkhorn coral remaining in the region.

Normally, she wears a wetsuit to protect against the cold, but that day she didn’t need one—the ocean was unusually warm. “The water didn’t feel refreshing,” she recalled.

As she peered through her diving mask at a reef that had taken centuries to grow, she saw that every single coral had turned ghostly white, their tissue sloughing off in chunks.

“I was seeing the coral actively dying, losing the tissue. And once that happens, there’s no coming back from it. I was crying underwater into my mask,” said Lesneski, a scientist who leads Florida’s Mission Iconic Reefs program. “It was very sad to witness, and I

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