Warning signs in the Pacific Ocean have had scientists on alert since July. The waters around Hawaii were so balmy that divers could swim without their usually obligatory wetsuits. Once-bright coral are losing their color. It all points to a terrifying prospect: the “blob” that rolled in nearly five years ago bringing death in its wake just might be back.

The original “blob” was an ocean heatwave that got its name for the splotch of red it made on maps in 2014 and 2015. Scientists had never seen anything like it before. It was massive, spanning the Pacific from Mexico to Alaska. Ocean surface temperatures rose as much as 7 degrees Fahrenheit above average. In the reefs surrounding Hawaii, that was enough to kill between 50 and 90 percent of corals.

“We were totally unprepared. We were naive as a science community.”

The scale of death was drastic, but there’s still some uncertainty over just how much was lost. “We were totally unprepared. We were naive as a science community,” says Greg Asner, director of Arizona State University Center for Global Discovery and Conservation Science, who is based in Hawaii. His team chases coral bleaching events the same way storm chasers track tornadoes and hurricanes. Coral bleaching occurs when stressed corals lose their color and often die, and this damaging phenomenon is becoming increasingly common. Half of Australia’s Great Barrier Reef has died off since 2016. The world has lost nearly a quarter of all its coral reefs in the past 30 years. The measurements Asner’s team takes will help calibrate the first satellites to track coral bleaching from space. And that could be key to saving what’s left and even restoring what’s gone.

Asner talks with his hands, using broad gestures when he’s excited about his project and when speaking to the urgency of the problem his team is addressing. He first set out to use satellites to map coral bleaching about two years ago, and those satellites started monitoring the bleaching event in July. His tool relies on daily pictures taken by more than 40 satellites operated by Earth-imaging company Planet as they pass over the reefs.

“We figured out a way to basically digitally peel back the seawater and see the seafloor,” Asner explains. The new satellite mapping tool, he says, is “basically a super fancy hue and tone change detector.” When the seafloor changes color, becoming lighter or darker as corals bleach, the satellites detect that change.

“I need people, I need more eyes on the reef, I need citizen science.”

As the satellites snap photos, Asner and his team verify ongoing bleaching events by going out into the field. But it’s not a huge team; there are about 30 core researchers. To be successful, Asner knew he needed more people on board, and they couldn’t just be scientists. His team’s efforts had started to feel like a global goose chase. When they heard rumors of coral bleaching in French Polynesia in May, they deployed immediately, only to find that they were too late, much of the coral had already bleached by the time they arrived. To get the most out of their upcoming satellite project, they needed to see what was happening from the start.

So in July, with murmurs of another blob striking Hawaii, Asner’s lab built a way for anyone in the water, scientist or not, to report what they were seeing. “Here comes a heatwave to my home state, and I thought, ‘Okay, if we don’t get this right here, we’re just never going to figure this out,’” Asner says. “All this science, all this great technology was not going to be good enough. I thought, ‘I need people, I need more eyes on the reef, I need citizen science.’”

The first step was to build a tool that just about anybody can use. Asner’s isn’t the first tool aimed at getting the public involved in monitoring the coral, but it is the most user-friendly. Other tracking efforts ask users to fill out more complex information, like the longitude and latitude of where they were swimming. His lab created a website that allows users to report bleaching by dragging a pin on a map to wherever they spotted the damage. They can indicate whether the bleaching they saw was light, medium, or severe, (with instructions on how to tell the difference) and then submit.

“What’s really tragic is seeing a very large head of coral that’s dead,” says Doug Perrine, a photojournalist in Kona who specializes in ocean photography and has used the website. He estimates that some of the larger corals took nearly 600 years to reach their size. On his dives, he’s seen destruction and very fragile hope — tiny growths of bright coral against the giant carcass of bleached coral colonies. But on a recent dive, he noticed that one of the tiny regrowths was starting to bleach, too. The reef was trying to recover, he says, but there wasn’t enough time between heatwaves for it to grow.

In September, the National Oceanic and Atmospheric Administration (NOAA) announced that the new blob was already the second largest marine heatwave in the past 40 years. It covers an area about seven times the size of Alaska. It hasn’t hit temperatures as high as they got in 2015, but unusually hot conditions have persisted for longer.

“In 2015, we burned the dish on the stove. This year is turning out to be different; it’s more like a simmer,” Asner tells The Verge. “The corals can’t handle the hot flash version of 2015 or the long simmering that they’re in now. They’re dying now.”

Corals thrive because of a symbiotic relationship with photosynthetic algae on their bodies that give the reefs their color and provide oxygen and nutrients. Near Hawaii, that algae paints reefs in greens, yellows, and browns. When the water gets too hot to handle, the algae leaves (scientists aren’t exactly sure yet where they go), and the reefs are left stark white. If that bleaching persists for too long, the coral eventually dies.

An oceanographer with NOAA tells The Verge that the peak of coral bleaching is likely unfolding now and into the next couple of weeks, making it a critical time for people like Perrine to report what they’re seeing. Asner’s team made sure to get the word out about their tool through social media, flyers, and television spots. The Arizona State University team also partnered with NOAA and the Hawaii Division of Aquatic Resources to launch the initiative.

“Reefs are just a canary in the coal mine.”

It seems to be working so far, although there are a lot more swimmers to reach. They’ve already gotten nearly 500 reports of bleaching across Hawaii, and users seem to like the interface. But Perrine told The Verge that he’d expect to see 500 divers out snorkeling in one reef on one day. He doesn’t remember exactly where he came across Asner’s coral bleaching tracker — he thinks it was probably on Facebook where he posts photos from his dives — but he has shared it with fellow underwater photographers like Jeff Milisen who is also based in Hawaii and who has started using the tool to report damage on his dives.

One of Milisen’s favorite spots to dive had been a “cleaning station.” These areas are sort of like spas where big fish like manta rays hang out, enlisting the services of smaller critters that remove parasites from their bodies. But since 2015, Milisen says, he hasn’t seen the rays there. Coral reefs are some of the most biodiverse ecosystems on the planet, so when they go — so too do the fish and other sea creatures who congregate around them.

“Reefs are just a canary in the coal mine” when it comes to climate change, Kuʻulei Rodgers, a researcher at the Hawai’i Institute of Marine Biology, tells The Verge. (Her university helped to develop another tracking tool that takes more detailed observations.) Coral bleaching events used to occur every 25 or 30 years, and now they’re happening every six years or less. Marine heatwaves are a new normal that even the United Nations is drawing attention to. “We have very little time. And we have only one chance to get this right, so it’s going to be important that we do everything we can come up with every kind of solution and tool that we can to try to tackle this, and to understand the bleaching,” Rodgers says.

Understanding the bleaching might actually hold the key to protecting coral reefs in the future. Information from people like Perrine, Milisen, and Asner’s field teams help direct the satellite cameras, and that information is now fed into a near-real-time map online that shows coral bleaching as it happens. The project received funding as part of a $1.5 million initiative backed by late Microsoft co-founder Paul Allen’s holding company Vulcan.

“Assessing coral bleaching real time via satellite is incredibly novel, and really incredibly revolutionary,” says Jamison Gove, an oceanographer with NOAA. It helps his agency and other scientists get the bigger picture of coral bleaching across the region and in hard-to-reach-places. It also helps the state figure out where it needs to put protections in place to relieve additional stressors, like regulating fishing and tourism.

The satellites’ most important role isn’t necessarily to document destruction, but to find where there’s resilience. They’ll look for coral that lived through heatwaves in areas with the worst bleaching. Those survivors are probably more tolerant of higher temperatures, and Asner calls them “the future genetics of the reef.” The state of Hawaii is experimenting with growing coral in nurseries and then bringing them out into the wild. More resilient coral, which they might be able to identify using Asner’s tools, could be the building blocks of these restoration efforts.

The maps have other uses, too. When it’s time to bring coral babies out of the nursery to face the real world, it could be best to introduce them to areas where they have the highest chance of survival. The satellites can also identify places act as refuges where coral resisted the heatwave. Those spots might be getting an infusion of cooler freshwater flowing out from the islands or they might be more free of human-induced stresses, like fishing. If researchers can identify the factors that protect these spaces, they might be able to learn lessons that could be applied elsewhere.

“I don’t want a job where I’m just documenting the demise and death of one of the most diverse ecosystems on the planet,” Gove says. “I think it’s important to highlight that there were corals that survived the 2015 bleaching event and there will be corals that survive this bleaching event.”

Even after successfully getting the satellite mapping tool up and running, Asner and his network of scientists and volunteers continue their exploration of Hawaii’s warming reefs. “Now we’re in the exhaustion phase; that adrenaline is over,” he says. “We’re just working, working, working.”

Even during his breaks, Asner keeps moving. When he needs a moment to think, he paints the sea-beaten steps to the marine center he put up in the remote fishing village of Miloli’i. He set up shop there to be near Hawaii’s last big intact reef. That’s where Milisen found Asner last month after a diving trip nearby, telling him, “I just wanted to come over and shake your hand.”