Only the scattering of light penetrates deep water, more than 300 feet deep 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 is difficult, expensive and dangerous to access. It can only be accessed by submarine, remotely operated vehicle, or by specially trained technical divers.
But humans are beginning to uncover the secrets of life here. In November, a group of scientific divers from the California Academy of Sciences successfully completed a series of risky deep dives.
Their mission was to bring back monitoring devices located on Guam’s deep coral reefs, which had been collecting data on marine life and ocean temperatures for more than eight years.
Divers have found a stretch of ocean inhabited by strange and wild creatures, from delicate corals and wriggling worms to spiny sea slugs and hairy crabs. Observers also provide insight into temperature changes, suggesting that climate change may be having an impact even in these deep waters.
Divers ascend after monitoring devices were installed in Guam in 2018. Only specially trained divers can access the ocean’s upper twilight zone. – Louise Rocha/California Academy of Sciences
Diving into the ocean’s upper twilight zone is a treacherous endeavor. Standard equipment simply isn’t enough at these depths—recreational divers breathing compressed air don’t venture below 130 feet. Instead, trained technical divers need specialized equipment that allows them to breathe a mixture of helium and air.
The deeper divers go, the more of the gas dissolves in their bodies, and it must exit very slowly to avoid decompression sickness, known as the bends, in which bubbles form in the blood that cause severe pain and even death.
The only way to avoid this is to climb very slowly, stopping every 10 feet or so. “If we stayed just 10 minutes at 500 feet, it would take us six hours to climb,” said Luiz Rocha, a diver and curator of ichthyology at the California Academy of Sciences. This means they rarely get more than 30 minutes to do their work.
But over the course of eight dives in November, Rocha and the team were able to safely collect 13 monitors. These curvy structures are made of one-square-foot PVC panels that act as artificial reefs on which animals can settle and grow. “They’re basically little underwater hotels that are colonized by coral reef organisms over time,” Rocha said.
The surface of one of the coral monitors collected by divers from a deep coral reef on Guam. – California Academy of Sciences
The monitors captured it in November 2025 after spending eight years on Guam’s deep coral reef. – California Academy of Sciences
The recovered displays — which look like works of art, filled with vivid colors of amber, burnt orange, deep blue and pink — were brought to the surface and transported to the University of Guam Marine Laboratory for processing.
There, scientists sifted through the encapsulated materials, identified each individual sample, photographed it, and collected it. Everything else was scraped off and sent for DNA analysis.
After two weeks of processing, 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 likely newly discovered species.
Bellomnid crab. – California Academy of Sciences
A possible new species of sea slug in the genus Kratina. – California Academy of Sciences
Possible new species of cardinal fish. – California Academy of Sciences
They found a possible new species of cardinal fish, a type of orange-clawed crab that had never before been reported on Guam, and a new species of sea slug, spotted in yellow and pink.
One of Rocha’s favorite finds was a hermit crab that uses clam-like shells as its habitat. “Every hermit crab I’ve ever seen uses gastropod (snail-like) shells as homes, but this species has some interesting adaptations that allow it to use shellfish instead,” he said.
But in addition to the exciting discoveries, there are also concerns about the future of deep coral reefs. More than half of the species that live here are unknown, Rocha said, “yet these reefs are already being affected by fishing, pollution and climate change.”
A young Pacific crown-of-thorns starfish was collected on a coral reef while diving in shallow water at a depth of about 80 feet. – California Academy of Sciences
A small species of octopus was found among the animals collected from the monitoring device. – California Academy of Sciences
Recent research into plastic pollution on coral reefs has found that the amount of plastic debris, most of which comes from the fishing industry, It increased with depth, It peaks in the upper twilight zone. “We are always the first humans to set eyes on these deep coral reefs, and yet we see human-generated trash on every dive,” Rocha said.
Climate change poses another major threat. Temperature data is still trickling in slowly from monitors, but scientists already see that deep waters may follow the same warming trends seen elsewhere. “This contradicts the assumptions that this depth would be a safe refuge and protected from rising temperatures,” Rocha said.
The expedition to Guam in November marks the start of a two-year process to bring together a total of 76 observers from deep on reefs across the Pacific, including Palau, French Polynesia and the Marshall Islands. Scientists hope this will give them a more detailed picture of life in the twilight zone, and how to protect this mysterious part of the ocean.
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