Scientists dove hundreds of feet into the ocean and found creatures no human has ever seen. Our trash beat us there

Only a scattering of light penetrates the deep waters, more than 300 feet below the ocean surface off the coast of Guam. To the human eye, it’s like dusk, even in the middle of the day.

This part of the ocean, known as the upper twilight zone, is one of the least explored ecosystems on Earth due to its difficult, expensive and dangerous access. It is only accessible by submarine, remotely operated vehicle or specially trained technical divers.

But humans are beginning to unravel the mysteries of life here. In November, a group of scientific divers from the California Academy of Sciences successfully completed a series of dangerously deep dives.

Their mission was to recover monitoring devices installed in Guam’s deep reefs, which have been collecting data on marine life and ocean temperatures for more than eight years.

Divers discovered a swath of ocean populated by strange and wild creatures, from delicate corals and shimmering worms to spiky sea slugs and hairy crabs. The monitors also provide insight into temperature changes, suggesting that climate change could have an impact even in these deep waters.

Diving in the upper twilight zone of the ocean is a perilous endeavor. Standard equipment simply isn’t enough at these depths: recreational divers breathing compressed air don’t venture below 130 feet. Instead, qualified technical divers need specialized equipment that allows them to breathe a cocktail of helium and air.

The deeper divers go, the more gases dissolve in their bodies and must exit very slowly to avoid decompression sickness, called bends, where bubbles form in the blood causing severe pain or even death.

The only way to avoid this is to climb very slowly, stopping every 3 meters or so. “If we stay just 10 minutes at 500 feet, it would take us six hours to come back up,” said Luiz Rocha, one of the divers and a curator of ichthyology at the California Academy of Sciences. This means they rarely have more than 30 minutes to do their job.

But in eight dives in November, Rocha and the team managed to safely recover 13 monitoring devices. These ridged structures are made of 1-foot-square PVC sheets that function as artificial reefs on which animals can settle and grow. These are “basically little underwater hotels that coral reef organisms colonize over time,” Rocha said.

The recovered monitors – which look like works of art, splashed with vibrant hues of amber, burnt orange, deep blues and pinks – were brought to the surface and transported to the University of Guam Marine Laboratory for processing.

There, scientists sifted through the embedded materials and identified, photographed and collected each individual specimen. Everything else was scraped and sent for DNA analysis.

After two weeks of processing, scientists have so far found 2,000 specimens, of which 100 were recorded for the first time in the region and 20 of them are potentially newly discovered species.

They discovered a possible new species of cardinalfish, a species of orange-legged crab never before reported on Guam, and a new species of sea slug, spotted yellow and pink.

One of Rocha’s favorite discoveries was a hermit crab that uses clam-like shells as a habitat. “All the hermit crabs I’ve seen before used gastropod (snail-like) shells as habitat, but this species has some really interesting adaptations that allow it to use clams instead,” he said.

But aside from these exciting discoveries, there are also fears about the future of deep reefs. More than half of the species that live here are unknown, “yet these reefs are already being affected by fishing, pollution and climate change,” Rocha said.

Recent research into plastic pollution of coral reefs found that the amount of plastic debris, most of which came from the fishing industry, increased with depth, peaking in the upper twilight zone. “We are almost always the first humans to lay eyes on these deeper reefs, and yet we see human-produced trash on every dive,” Rocha said.

Climate change poses another major threat. Temperature data is still slowly arriving from monitoring devices, but scientists are already seeing that deeper waters could follow the same warming trends seen elsewhere. “This contradicts assumptions that this depth would provide a safe refuge, protected from warming,” Rocha said.

The November expedition to Guam marks the start of a two-year process to collect a total of 76 monitor lizards from the depths of Pacific reefs, including Palau, French Polynesia and the Marshall Islands. Scientists hope this will give them a much more detailed picture of life in the Twilight Zone and how to protect this mysterious part of the ocean.

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