Only a scattering of light penetrates the deep water, more than 300 feet beneath the ocean surface off the coast of Guam. It’s like twilight to the human eye, 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 because it’s so difficult, expensive and dangerous to reach. It’s only accessible by submarine, remotely-operated vehicle or by specially trained technical divers.

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

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

The divers found a swath of ocean populated by weird and wild creatures, from delicate corals and shimmering worms to spiky sea slugs and hairy crabs. The monitors are also providing an insight into temperature changes, suggesting climate change may be having an impact even in these deep waters.

Diving to the ocean’s upper twilight zone is a treacherous endeavor. Standard gear simply doesn’t cut it at these depths — recreational divers breathing compressed air don’t venture below 130 feet. Instead, trained technical divers need specialized equipment allowing them to breathe a cocktail of helium and air.

The deeper the divers go, the more gas gets dissolved into their bodies, and it needs to come out very slowly to avoid decompression sickness, known as the bends, where bubbles form in the blood causing severe pain and even death.

The only way to avoid this is to ascend very slowly, stopping every 10 feet or so. “If we stay just 10 minutes at 500 feet, it would take us six hours to go up,” said Luiz Rocha, one of the divers and an ichthyology curator at the California Academy of Sciences. It means they rarely get more than 30 minutes to do their work.

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

The retrieved monitors — which look like works of art, splashed with vivid hues of amber, burnt orange, deep blues and pinks — were taken to the surface and rushed to the University of Guam Marine Laboratory for processing.

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

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

They found a possible new species of cardinalfish, a species of orange-clawed crab never before reported in Guam and a new species of sea slug, speckled with yellow and pink.

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

But as well as the exciting discoveries, there are also fears for the future of the 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 on coral reefs found 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 set eyes on these deeper reefs, and yet we see human-produced trash on every dive,” Rocha said.

Climate change is another big threat. Temperature data is still slowly trickling in from the monitoring devices, but the scientists are already seeing that deeper water may be following the same warming trends seen elsewhere. “This contradicts assumptions that this depth would be 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 monitors from deep in the reefs across the Pacific, including in Palau, French Polynesia and the Marshall Islands. The scientists hope this will give them a much more detailed picture about life in the twilight zone — and how to protect this mysterious part of the ocean.

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