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On an ocean ridge 300 feet beneath the island of Maui, one species of coral has found a place to hide—from global warming.

Ohio State oceanographer Andrea Grottoli joined her colleagues Brian Popp and Kenneth Longenecker at the University of Hawaii. Together, they pushed the limits of technology to visit this newly discovered coral community, and find out how it manages to survive so deep in the water, and so far away from sunlight.

Today, nearly a third of the world's coral reefs are on the verge of extinction. Reefs provide habitat for hundreds of marine species, protect coastlines from erosion, and provide vital income to many countries that depend on tourism.

But as shallow waters become hotter and more acidic from global warming, deep ridges like this one could become coral's last refuge from the changing climate above, Grottoli explained.

Still, coral need sunlight to survive, so she didn't expect to find much 300 feet down – perhaps a few blooms of Leptoseris hawaiiensis, or "plate coral," scattered here and there.

But when her underwater submersible crested the ridge for the first time, she couldn't believe her eyes.

"It was just a lawn of Leptoseris as far as you could see," she said.

The experiment that followed went like clockwork – it had to.

"It's unprecedented for any kind of research to combine diver and submersible," Grottoli said. "It was challenging from a coordination perspective."

The site was at the shallowest depth a submersible would work, but the lowest depth for human divers, who could stay below for only 20 minutes at a time. Yet both were needed if the researchers were to set up their equipment and take samples of the coral in time.

This work is just the latest in a series of experiments all over the world in which Grottoli is working to understand how coral might adapt to climate change. Most species can't tolerate the changes that are already happening to our environment, but occasionally she finds one that has changed its body chemistry or growth patterns to persevere.

Like us, coral are animals, but unlike us they get most of their energy from algae that live in their cells. The algae photosynthesize to make food for the coral, so sunlight is critical for most corals' survival. But coral also can feed directly on plankton that they capture with their tentacles.

Now that the Maui samples are back in Grottoli's lab, she and her students will discover how these particular plate coral survived.

The team includes undergraduate students Sarah Starkey, Dana Borg, and Justin Bauman, and doctoral student Verena Schoepf. A few months from now, they hope to have their answers, but right now an early picture is emerging.

"Our guess is that they use far less photosynthetic energy compared to their shallow-water counterparts," Grottoli said. The coral probably eat more plankton to make up for the lack of sunlight.

To see what the future holds for the world's shallow-water coral, the researchers are working with a coral reef farm in New Albany, Ohio. There, they can see what happens when coral are repeatedly exposed to high temperatures and acidic water at the same time.

Scientists expect that by 2050, all coral reefs will be on an irreversible decline.

So far, Grottoli's research suggests that a handful of coral species may be able to survive, but they almost certainly won't thrive.

"It's a bleak outlook," she said, "but there are some rays of hope."

Special thanks to Richard Pyle for providing field video and to the National Science Foundation for its research support.

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