Diving Into the Twilight Zone

Through centuries of exploration, humans have climbed the highest peaks and hacked through the densest jungles. From pole to pole, there isn’t a continent left unexplored, and very little land on earth that has not been set foot on by a human being. Yet only 10 percent of the world’s vast oceans have been truly explored. And among the least explored regions of our watery world are the mesophotic reefs, an area that scientists refer to as “the twilight zone.”

At 150 to 500 feet deep, this region is too deep to safely reach using conventional scuba, but thought too shallow to justify the use of expensive submersibles.

“To understand what is really down there, you need to actually go there -- you need to witness it and collect,”said Bart Shepherd, the director of the Steinhart Aquarium at the California Academy of Sciences in San Francisco. “Exploring the mesophotic zone with a Remotely Operated Vehicle (ROV) would be like exploring the rainforest with a remote control helicopter.”

But with recent advances in diving methods and technology, such as closed-circuit rebreathers, mixed gases and propelled scooters, trained divers can now venture into these deeper reefs. Recently a team from the California Academy of Sciences made up of Shepherd, diving safety officer Elliott Jessup, and ichthyologist Luiz Rocha, began to explore this mysterious place.

“The biggest challenge with that depth is just getting there and getting back,” said Shepherd. “It’s not unusual for us to have a dive last five hours long. But that only gives us 45 minutes below 160 feet.”

According to Shepherd, the divers must slowly ascend from the twilight zone, systematically stopping at depth intervals, to avoid contracting the bends.

“In order to penetrate down to the depths of the twilight zone and have enough time to work and collect, we need a lot of decompression time,” he said. “But even during that decompression time, we are exploring.”

Each time they dive, the researchers are seeing and discovering places and things never before seen by human eyes. It’s been estimated that these scientists are discovering nearly a dozen new species per hour in the twilight zone.

In May 2014, the twilight zone dive team was part of a large, month-long expedition to the Philippines. Researchers from all over the world joined Academy scientists to study the Verde Island Passage. This relatively small area of the ocean south of Manila has been referred to as the “center of the center or marine biodiversity.” In fact, it is thought that more unique species live there than on the entire Great Barrier Reef. The scientists, along with fishery managers in the Philippines, want to better understand why this region is so unique and resilient.

“Why is it such a biodiverse place?” said Shepherd. “It certainly has to do with the location in the Pacific. It’s in the center of what we call the Coral Triangle. And it probably has to do with the proximity to the deeper water there. There’s a lot of nutrients that come up out of that deep water and sort of feed that marine community. There’s just an incredible amount of diversity. And it’s certainly very interesting to think about why there.”

Scientists from the California Academy of Sciences have been studying the Philippine marine environment and documenting their findings for 20 years. They have found huge numbers of reef fishes, corals and other marine species.

“It just seems like that spot has more types of animals living in these really, really rich and dynamic communities than anywhere else on the planet,” he said.

Add that the deeper regions have never been explored there, and the possibilities for new discovery jump to mind-boggling levels. Or as one expedition diver said, “Science is adventure. Science is discovery. There is a lot more science to do here!”

Additional Links

• Watch the full episode of QUEST.