Groundbreaking dives off Easter Island — or Rapa Nui — reveal a treasure trove of species discovery ...

Somewhere between the limits of conventional scuba diving and the depths explored by ROVs and submarines, we travel through another dimension. It’s a wondrous land of discovery. Our next stop: the twilight zone. But rather than the stuff of science fiction, this is more likely to be found in science textbooks.

Rapa Nui, or Easter Island, a Chilean territory in the South Pacific, is known for its history of ecological collapse due to overpopulation and the exploitation of its natural resources. Iconic human carvings known as moai (constructed 1300–1600 A.D.) still stand sentry over the landscape and attract tourists from around the world.

Few people know that Rapa Nui is a fantastic diving destination, its subtropical waters offering some of the best visibility I’ve ever seen. But the type of diving popular with tourists is still a far cry from what our team had planned.

We’re researchers from the California Academy of Sciences on expedition for our Hope for Reefs initiative — a monumental effort to explore, understand and restore coral reefs around the globe. In collaboration with Chilean colleagues from the Pontificia Universidad Católica de Chile, we surveyed Rapa Nui’s marine diversity from the shallows to the deep “twilight zone” (or mesophotic) reefs. These ecosystems are found 200 to 500 feet below the surface. Few scientists are willing to invest the time and resources into the technical training necessary for rebreather diving to those depths, leaving the mysterious twilight zone relatively unexplored. Preparing for the first-ever technical dives in this area meant overcoming many logistical hurdles. We sent our bulkier gear — scooters, large cylinders of helium and oxygen, sorb for the rebreathers, and bailout tanks — by ship. We also packed a portable Hyperlite decompression chamber that collapses down to two suitcases and, when operational, can take one diver to a depth of 60 feet on 100 percent oxygen. The chamber can be inflated inside an airplane should we need to transport a patient to more robust care.

After much preparation, we head for our first dive on a small island called Motu Nui, famous for the Rapa Nui Birdman tradition. Motu Nui is also known for steep underwater walls (which often mean good deep diving). Since we spend up to six hours decompressing, it’s always nice to be near a wall where we can continue taking fish counts along transects, documenting coral disease, estimating levels of plastic pollution and taking pictures. Unfortunately, we find that at 260 feet deep the wall becomes a flat, sandy bottom.

As we search for better sites within our target depth range of 300 to 400 feet, we use a depth sounder to scout locations before dropping into the blue. Hit or miss is an understatement. The amount of gas that our bodies absorb at those depths is high, so if we don’t find anything interesting at the bottom on our first try, we are done for the day. And that’s how the first few dives go: sand, sand and more sand.

With only three expedition days remaining, we finally hit the sweet spot and find rocky formations at 360 feet. I immediately see a new species of Anthiadinae fish. Males and females typically have different colors, so we collect both. After surfacing, we realize we actually have two different species on our hands — each entirely unknown to science. We also collect a new species of the genus Plectranthias and a new Chromis damselfish. Discovering four new species of fish in a single dive is incredibly exciting! This much novelty, while not unusual for the twilight zone, was unexpected in Rapa Nui, because it has the lowest diversity of any coral reef system in the South Pacific.

The island’s extreme isolation means few species arrive in larval form, making diversity low and new discoveries rare. We also observed clear signs of human impact: Broken fishing lines, old anchors and cables were commonplace underwater. I was saddened to see such evidence in a place that very few have even laid eyes on. We know that these twilight zone ecosystems are incredibly unique, so we hope to serve as ambassadors by collecting important data and fostering hope for their future protection.