Three and a half miles off the Florida Keys, Kylie Smith kicked to the surface of the ocean, then looked back down through her scuba mask to confirm what she had seen in the coral reef 15 feet below.

A colony of baby elkhorn coral was bone white. Another grouping of the animals was dying from rapid tissue loss disease. Smith’s team of volunteers had planted the baby coral in the last year and watched them thrive. Now both colonies — which had slowly spread, with tiny nubs reaching toward filtering sunlight — were being damaged as water temperatures spiked.

“It’s heartbreaking,” the 34-year-old coral ecologist said softly, cupping her hands around her scuba mask as she bobbed in the waves above Alligator Reef.