There's still so much to learn about what's under the 70 percent of Earth's surface covered by saltwater. When I recently wrote about how we know more about the surface of Mars than what's under our oceans, I was shocked to find out how unexplored our ocean's depths really are. We have a decent idea of what's near the shore, and we have monumental seamounts and giant canyons mapped, but that's about it.

The latest incredible example proving just how much we don't know is a newly discovered mesophotic coral reef located off the coast of Maui in the Hawaiian Islands. The term mesophotic means deep, so this coral lives in the "twilight zone" between 100 and 300 feet deep. And it appears to be thriving, unlike much of the shallower coral around the world, which is dying off in record numbers due to coral bleaching. The reef extends three square miles, and much of that is 100 percent covered in coral.

“What is unique about this study is how vast and dense the coral cover is,” Richard Pyle, an author of the study and a zoologist at Hawaii’s Bishop Museum told The Associated Press.

The details of the reef, which is beyond the depth ranges of average human divers, were explored by a combination of expert divers, drop cameras, data recorders and robot vehicles. The results of the 20-year-long project, which involved botanists, geologists and biologists, was reported in the journal JPeer.

It's important to involve all these different methods of exploration because they all serve a different purpose: “[Humans] can do things that submarines can’t, like collect fish specimens and lift rocks and go inside caves,” Pyle told Wired magazine.

Hawaiian squirrelfish swim along a coral reef. Hawaiian squirrelfish swim along a coral reef. (Photo: U.S. Fish and Wildlife Service/Wikimedia Commons)

Outside of the incredible density of coral cover, the research team also found a high level of endemic (indigenous) fish species in some parts of the reef, with up to 100 percent local fish — the highest ever found anywhere on Earth. (In other areas of the Hawaiian Islands, endemic fish species make up less than 20 percent of surveyed fish species.)

Randall Kosaki, deputy superintendent for the Papahānaumokuākea Marine National Monument at the National Oceanic and Atmospheric Administration (NOAA), told the AP that these deeper reef systems' diversity is "off the scale globally" in terms of what they've seen elsewhere — even compared to other parts of the Hawaiian Islands.

There was a sense of urgency to the work: “There is a time sensitivity to this exploration because due to climate change and other factors, we’re at risk of losing species before we even know they exist,” Kosaki said.

These deep reefs may be serving as dark and quiet refuges for many fish species — some of which are the same types found in shallower reefs. This means at least the mobile residents of the sea have places they can escape from fishing nets, boat motors and the polluted water that runs off agricultural fields. (In Hawaii, chemicals from sugarcane fields are especially hazardous to reefs.)

There's still more to learn about these reefs — including where others might reside around the world. So far, human scientists have only scratched the surface of the "twilight zone."

Starre Vartan ( @ecochickie ) covers conscious consumption, health and science as she travels the world exploring new cultures and ideas.