A federal research vessel returned to Honolulu on Wednesday after discovering perhaps a dozen species never seen before in an expedition exploring the coral reefs of the Northwestern Hawaiian Islands at greater depths than ever before.

Among the discoveries was a possible new species of sea horse and a sea star previously not seen in Hawaii.

In addition, fish surveys at depths of up to 300 feet around the northernmost atolls showed a surprisingly high abundance of species found only in the Hawaiian Islands.

“Every dive we did was on a reef no human being has ever laid their eyes on,” said chief scientist Randall Kosaki, deputy superintendent of the Papahanaumokuakea Marine National Monument.

Twenty scientists from a handful of state, federal and private institutions spent 27 days aboard the National Oceanic and Atmospheric Administration ship Hi‘ialakai, exploring the marine conservation area encompassing the string of islands and atolls stretching to the northwest of the main Hawaiian chain.

The scientific team observed rarely seen ecosystems using advanced diving technology — or, as Kosaki put it, “scuba gear on steroids” — allowing divers to go much deeper than conventional scuba gear permits.

The primary mission of this expedition was to reveal the unexplored habitats of the national monument’s open-ocean seamounts, which were first mapped using high-resolution multibeam sonar in 2014 and 2015.

The seamounts — underwater mountains that were formerly ancient islands — rise from the floor of the ocean in 14,000 feet of water and climb to within 200 to 300 feet of the surface.

At those depths, daylight starts to disappear and the temperature drops to the high 50s, the scientists said. The underwater environment revealed itself only with powerful lighting, allowing the diving scientists to collect specimens and take images of the hidden marine life.

“It was exciting,” recalled Richard Pyle, database coordinator for natural sciences at Bishop Museum, “because you never know what you’re going to find when you go to a place no one’s ever been to before.”

The scientists observed potential new species of fish, algae and invertebrates. Some of the fish, while not new, were never before seen by divers and are known only from fishing nets, they said.

The specimens collected on the cruise will be sent to experts at museums around the world to confirm the identity of the organisms. It will take several months of lab work and analysis to figure out whether in fact they are new species, officials said.

Over the past five years, this annual NOAA research cruise has brought back more than 70 confirmed new Hawaiian species, Kosaki said.

During the most recent voyage, the ship spent more than a few days dodging tropical storms. Three or four cyclones threatened the area during the trip.

“Even if the storms weren’t a direct hit, they generated swells that affected our diving,” Kosaki said. “We still managed to get a large majority of the work done.”

Pyle said the team wasn’t planning to spend so much time around Kure Atoll, but the scientists were glad they did. He said they found deep reefs that were home to an astonishing 100 percent endemic species — creatures unique to the Hawaiian archipelago.

“Originally, we weren’t planning to go to Kure,” Pyle said. “The weather drove us there, but then the high rates of endemism kept us there.”

Kosaki said it was the highest level of endemism recorded from any marine ecosystem on Earth.

“We were very surprised that these deep reefs are completely dominated by fishes found nowhere else in the world,” he said. “One hundred percent endemic was off the scale and totally unexpected.”

In addition to studying the marine environment, the diving scientists were also the subjects of a medical study to understand the effects of extreme dive exposures on the human body.

Dr. Neal Pollock, research director of Divers Alert Network, led an onboard team that conducted ultrasound imaging of the divers’ hearts following the deep dives.

Formation of gas bubbles in the bloodstream on ascent from deep dives is known to cause decompression sickness, also known as the bends, a potentially life-threatening condition.

While no one was hurt during the voyage’s deep dives, Pollack said he’s hoping the data will better inform the algorithms used by dive computers to guide divers through gradual decompression from deep dives and make scuba diving safer for scientific divers as they go even deeper in the future.

The scientific team included researchers from NOAA’s Office of National Marine Sanctuaries, NOAA’s Pacific Islands Fisheries Science Center, the University of Hawaii’s Hawaii Institute of Marine Biology, Bishop Museum and the Divers Alert Network.