Many coral reefs are dying. This one is exploding with life.

Scientists recently observed a rare phenomenon on a reef in Cambodia that left them in awe — and filled them with hope.

Once a year, after dark, a bit of magic happens in the ocean.

In tropical waters worldwide, large chunks of coral — those colorful rocklike structures in shallow, coastal waters, each a colony of living animals — start puffing out hundreds of little pearl-sized balls. Some are pink. Others are red, orange, or yellow. For a few minutes, the ocean is a snow globe, and then the balls float away.

This phenomenon, known as spawning, is how many corals reproduce. Each ball is a bundle of eggs and sperm from an individual coral colony. Different colonies of the same species somehow know how to spawn on the same day and same time, so their eggs and sperm can meet and form baby corals.

Spawning is incredibly hard to observe. Again, it happens only once a year, and often only for a few minutes at night. Plus, stressed or damaged corals are less likely to spawn. And across large swaths of ocean, extreme heat — linked to climate change — has been stressing these animals out.

But earlier this month, a team of marine biologists got lucky: They witnessed a massive spawning event off the coast of Cambodia, in the Gulf of Thailand. Not long after sunset, several different kinds of coral filled the water with pearls.

The team, led by Fauna & Flora International, an environmental group, was able to capture the event on video, shown in a series of clips below.

“It was like it was snowing,” Tharamony Ngoun, a marine species and ecosystems officer at Fauna & Flora, who observed the spawn, told Vox. “It was so amazing.”

Spawning on Cambodia’s reefs is not only thrilling to witness (I’ve been lucky enough to see coral spawning before, though not in Cambodia). It also offers hope for these important ecosystems, many of which are under siege.

Globally, coral reefs, which underpin commercial fisheries and protect coastlines from storms, have declined by half since the 1950s, largely due to climate change. Spells of extreme marine heat break down the relationship between coral and a type of symbiotic algae that gives it both food and its vibrant colors. The coral turns white — a process referred to as bleaching — and can then easily starve to death.

The reefs in Cambodia and in the broader East Asian region, however, appear to be bucking this trend. Surveys indicate that they haven’t declined in recent decades, perhaps because they’re more resilient to warming. Their secret to survival may ultimately help safeguard ailing reefs elsewhere.

The outlook for coral reefs is not good

The last 12 months have been particularly bleak for coral reefs.

Last summer, a record-breaking marine heat wave turned water in the Florida Keys — home to one of the world’s largest barrier reefs — into a hot tub, causing bleaching on a massive scale. Before and after photos show hundreds of corals, once appearing in bright oranges, blues, and greens, in stark white. The National Oceanic and Atmospheric Administration (NOAA) recently assessed the damage, finding that only a fraction of certain threatened species are still alive.

Now, as summer drags into fall in the Southern Hemisphere, large sections of the Great Barrier Reef have been bleaching, too, following near-record levels of heat stress. Scientists have also warned about bleaching elsewhere this year.

“This latest, still-unfolding event was entirely predictable, as ocean temperatures continue to rise due to global heating,” Terry Hughes, one of the world’s top coral reef experts, wrote about the Great Barrier Reef earlier this month.

Indeed, the leading scientific authority on climate change suggests that if the world warms by 1.5 degrees Celsius, relative to pre-industrial times, coral reefs could decline by 70 percent to 90 percent. And we’re basically already there.

Southeast Asia’s reefs are hanging on

The dire outlook for coral makes this spawning event even more special. While many reefs are disappearing, others are relatively healthy and capable of producing a new generation of corals.

“The coral is thriving,” said Matt Glue, a marine technical specialist at Fauna & Flora, which sent a team earlier this month to try to observe spawning. “Everywhere we would go we would see more colonies that were spawning. It’s very hopeful.”

The reefs in Cambodia are not free of problems. Overfishing has diminished larger predators, like groupers, which help maintain the health of the reef. And although the amount of coral has remained relatively stable, contrasting global declines, it’s likely that more sensitive species have become less abundant and others more abundant, tweaking the makeup of the ecosystem.

Nonetheless, this reef does seem more resilient, according to Glue. While this region has experienced plenty of marine heat and some amount of bleaching, the heat typically doesn’t cause a mass die-off like what you see elsewhere.

The secret to the reef’s survival may be in the diversity of its corals. East Asia has a huge number of coral species and a lot of genetic diversity within individual species. The more varieties of coral a reef has, the more likely it is that some of them may have slightly more or less tolerance to various stresses, such as high temperatures. During a bout of severe warming, some coral colonies may die off, but others can take their place, Glue said.

Within East Asia, “high coral cover and diversity on the coral reefs within this critically important region may have conferred a degree of natural resistance to elevated [sea surface temperatures],” coral scientists wrote in a 2020 report.

What’s more is that these corals may help reefs elsewhere withstand the worsening wrath of climate change. Research has found that tolerance to heat is baked into the DNA of some coral colonies. And importantly, two heat-tolerant parents tend to produce heat-tolerant babies.

“If these corals are indeed unusually tolerant in whatever manner, the fact that they are actively producing larvae provides the direct possibility for those larvae to disperse to adjacent reef areas,” said Margaret Miller, one of the top coral experts in the US and research director at the conservation group Secore International. (She was not part of the team that observed the spawning.) In other words, all of that new spawn may help seed the ocean with more resilient corals.

“It feels really great to be part of this,” Glue said, of observing spawning with his team. “And hopeful — hopeful for the future of reefs in the Gulf of Thailand.”