The Hawaiian coral reefs you know—the brilliant blue waters and rainbow fishes and the occasional sea turtle—are just a facade. Sorry, but it’s true: Science says so. Dive past these upper-ocean paradises, deeper and deeper, and you’ll find even more incredible reefs—ones that, shrouded in near-total darkness, shouldn’t exist at all.

Hundreds of feet below the ocean’s surface, the so-called deep reef—also known as the twilight zone—is just coming into view. Today in the journal PeerJ, a multidisciplinary army of scientists has published a 20-year study of the deep reefs off Hawaii: geologists, biologists, and botanists working together to shine light on a world that’s upending the very idea of a coral reef. And—I’m sorry again—making that Hawaiian vacation of yours look tame.

It’s only been 20 years since scientists started learning about deep reefs. The research gap is a matter of logistics: The twilight zone is too deep to reach using traditional scuba gear, and too shallow, typically, to justify the $30,000 to $40,000 a day it costs to deploy a submersible. So scientists could only begin studying deep reefs thanks to the invention of what’s known as a rebreather, which recycles the (very expensive) helium that’s mixed in with a scuba tank’s oxygen. With this system, divers can manage to stay submerged for seven hours—necessary when you have to ascend slowly to avoid the bends.

In Hawaii, this team of scientists dove into the deeper reefs with a rebreather. They descended alongside submersibles (thanks to generous funding from NOAA, in no small part) that carried extra tanks and illuminated the depths, freeing up the divers to do the hands-on science. “We can do things that submarines can’t, like collect fish specimens and lift rocks and go inside caves,” says Richard Pyle, an author of the study and a zoologist at Hawaii’s Bishop Museum.

CoralReef5.jpgRichard Pyle

And, in one particularly interesting experiment, lift domes. Check out the third slide in the photo gallery at top. By placing a dome over coral and staining it, researchers could monitor its growth, a bit like tree rings. That whole apparatus was too heavy for divers to lift—so the submersible took care of the heavy lifting, while divers did the precision work.

That cooperation led to some bizarre discoveries. In a shallower reef in Hawaii, 17 percent of the fish species will be unique to that ecosystem—you won’t find them anywhere else on Earth. But in some of the deeper reefs around the islands, scientists were finding that that proportion went up to 50 percent. “Then moving up to the northwestern Hawaiian islands,” Pyle says, “there were some spots, particularly in Kure, down at 300 feet, where literally every fish on every survey is a species known only from the Hawaiian islands.”

Pyle has a theory for why this is, a theory that has implications that reach far beyond Hawaii. Up closer to the surface, a reef isn’t guaranteed to always be wet: Sea levels fluctuate wildly every 100,000 years or so. “That’s due to ice ages,” Pyle says. “So every time we have an ice age, a lot of the world’s water is locked up in glaciers, and as a consequence the sea level drops by about 300 feet.” This dries out those shallow reefs, killing everything that can’t flee to deeper waters.

But down in those deeper reefs, the ecosystem gets by just fine. So the twilight zones off the Hawaiian islands are probably ecologically ancient. So many endemic species of fish swim here because they’ve evolved here for so long. Young shallow reefs come and go, but deeper ones stay old and grizzled.

Old and grizzled is good, in a way. Old and grizzled means stability—exactly what this planet needs right now. As the oceans go haywire, as temperatures rise and waters acidify and food chains crumble and shallow reefs bleach, it might be that the deep reefs could serve as a kind of refuge. Perhaps as the shallow reefs continue to degrade, the species that live there can retreat deeper. At least those blessed with mobility.

What this study found is that yes, indeed, some species are able to live in both the shallow and deep reefs around Hawaii. “So if the same species are present, and if that population is healthy, it might help replenish it,” says NOAA’s Kimberly Puglise, a program manager in the study. “We don’t know if it can do that, but this is a step toward answering that question.” It’s a lot of uncertainty, sure, but it’s called the twilight zone for a reason.


Hawaii’s Mysterious Coral Reefs Turn Out to Be Super Weird