There was a time, just a few decades ago, when this crisis seemed unimaginable, when reefs seemed invincible. Phil Dustan, from the College of Charleston, similarly remembers being fresh out of grad school and telling the famed explorer Jacques Cousteau that “reefs are so big that humans couldn’t hurt them.” Those words seem hopelessly naïve now. Dustan recently dove at Dancing Lady Reef in Jamaica—a place that he had studied as a graduate student in the 1970s, and where scientists “first became intimate with the science of reefs,” he says. “I dropped into the water and I just choked. It was like someone going through their home after a forest fire has gone through, picking through the ashes.” Elsewhere in the Caribbean, he took his son snorkeling at Carysfort Reef, another site of once-legendary beauty. “He stayed real close to me and he wouldn’t range around because he was fearful,” Dustan says. “Finally, he said: Dad, we have to leave this place. It creeps me out. It’s all dead.”

This catastrophe has unfolded slowly. Nancy Knowlton, from the Smithsonian Institution, says that when it comes to corals, the bad news is usually incremental, and only obvious to those who work in the affected places. “But what happened in the Great Barrier Reef was so spectacularly bad that you didn’t need to work there to know it was bad,” she says.

What happened was this: In 2015, the world experienced a mass-bleaching event, where heat waves started killing corals in all three oceans where they thrive—the Pacific, Atlantic, and Indian. Two such global crises have happened before, in 1997 and 2010, but the 2015 one was unprecedented in its severity, and in its implications. Just 9 percent of the individual reefs that make up the 1,400-mile-long Great Barrier Reef, off the coast of northeastern Australia, were unscathed. All told, more than a quarter of the corals have died there, with a much higher proportion in the northern sections. No one had seen anything like it before. When the coral researcher Terry Hughes revealed the scale of the devastation to his students, they all reportedly wept.

“The news has been especially upsetting, because of the scale of the event, the iconic nature of the Great Barrier Reef, and the fact that there were gifted filmmakers on site to document it,” says Knowlton. “It was a perfect storm of attention.”

For many coral aficionados, it was also a tipping point for despair. One year of bleaching would be bad enough, but the Great Barrier Reef experienced a second in 2016 and there are signs that a third wave might hit by the end of this year. Corals bounce back, but consecutive blows could take even these resilient animals down for the count. “Our models said that wouldn’t happen for a long time, and I’m worried that we’ve underestimated the pace of change,” says Colton. “Things are even worse than we thought, and that’s been hard to cope with.” For her, that gnawing unease has led to sleepless nights, and curtailed her ability to deal with other worrying world events. “My barrel is full,” she tells me. “One more drop and it spills out. My resilience is gone.”