Cori Kane calls it “underwater skydiving.” She’ll be out in the middle of the North Pacific, more than 1,000 miles from Honolulu and most anything else that might be called civilization. Flopping out of a perfectly good boat, she will rocket down nearly 300 feet in just a few minutes, encountering a strange and largely unexplored layer of ocean that’s less familiar to science than the deep sea. It’s the ecosystem of the mesophotic reefs, which lie at a depth often called the “Twilight Zone.”

“When you jump in, it’s like you’re transported to this other world,” says Kane. “There are fish everywhere. There are big fish. There are sharks. There are these big species of jacks called ulua that are like packs of dogs that come and follow you and check out what you’re doing. Half of them are almost as big as I am. It’s one of the few places where I jump in the water and instantly realize I’m not the biggest fish in the sea. It’s an amazing experience.”

A self-described “fish nerd” and Washington State University doctoral candidate in marine biology, Kane spent five years diving in the vast Hawaiian archipelago as research coordinator for the Papahānaumokuākea Marine National Monument. It is one of the largest marine protected areas in the world, with some of the highest percentages of endemic fish—species found nowhere else. Over dozens of dives, Kane found even greater numbers of endemics as she and her colleagues counted and catalogued fish in the monument’s most remote regions.

“In a nutshell, we found a lot more than we ever could have imagined,” she says.

Created by President George W. Bush in 2006, the monument is remote by most any standard. It’s particularly far removed from Kane’s childhood in Illinois, where her parents built a house by first taking out six acres of corn.

“Even in the middle of the corn fields I was always poking around in the dirt and playing with all the animals I would find,” she recalls. Then her parents took her to SeaWorld, where her dad talked the staff into letting her be the kid that rides an orca during a show. She went on to get a bachelor’s degree in marine biology and a master’s in marine ecology, leading to her position at the monument.

For years, scientists have known that the shallower waters around Hawaii contain huge numbers of endemic fish. A little more than one in four are found nowhere else. But the deeper waters were off limits for exploration, as the dangers of nitrogen narcosis and decompression sickness limited traditional scuba divers to about 100 feet.

But starting in 1989, Hawaii ichthyologist Richard Pyle started developing gas mixes and techniques to dive deeper. Exploring the waters of the Cook Islands, Papua New Guinea, and the Patau Islands, Pyle found more than 50 new coral-reef fish species, estimating that another 2,000 await discovery. The new technology also let Kane and her colleagues start exploring the deeper reaches of the Northwestern Hawaiian Islands and make the first description of their mesophotic reef communities.

Kane recalls that after one of the earliest dives, Randy Kosaki, deputy superintendent of the monument, said, “Did you notice that almost all these fish are endemics?”

Kane, Kosaki, and Daniel Wagner, the monument science advisor, confirmed the impression by swimming transects at more than 50 sites between the main Hawaiian Islands and the Kure Atoll more than 800 miles away. Plummeting as deep as 280 feet, they would follow a line, identifying, counting, and estimating the size of fish a set distance to the left or right.

“It’s like taking a snapshot of the reef,” says Kane.

It turns out that nearly half the Northwestern Hawaiian Islands reef fish—46 percent—seen by Kane and her colleagues are endemic. That’s nearly twice as high as any other tropical region. At the northernmost end of the islands, at a latitudinal limit of coral production called the Darwin Point, the endemism ran as high as 92 percent.

“This unprecedented rate of endemism,” Kane and her colleagues wrote earlier this year in the Bulletin of Marine Science, “indicates that mesophotic reefs in the NWHI are reservoirs of biodiversity, and further underscores the need for protection of this area.”

“By protecting this area, we are protecting a huge chunk of the world reservoir of species, if you will,” says Wagner. “If we concentrate on protecting some of these hot spots, we have a lot of species and a lot of unique species, you can get more bang for your buck.”

Scientists can use the endemic fish to study subtle variations among species, teasing out their evolution much as Darwin did with the finches of the Galapagos Islands. The monument’s protections can also hedge against endemics’ particular vulnerability to extinction.

“If something happens to that population of endemic fish or that species,” says Kane, “there’s nowhere else to help it repopulate. It’s kind of one of those indicator species or priority species that you watch, especially when you’re concerned about a certain area, because it has the lowest chance of survivorship if anything super bad would happen.”

Kane says threats to the region include plastic pollution, lost fishing nets that can strangle the reef, and the rising ocean temperatures of global warming.

“As climate change goes on and the ocean temperatures rise, we may be seeing a decrease in the level of endemism or fishes disappearing completely because they can’t tolerate the warmer waters we may be transitioning into,” Kane says. “One of our main goals is to try and document as much as we can right now because we don’t even have a baseline for what these mesophotic reefs are. We could already be losing things and there could be catastrophic changes going on that we have no idea about, just because we haven’t been keeping an eye on them before now.”