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The "Octopus Garden" is the location of the largest gathering of octopuses anywhere on Earth, and new research has an explanation.

Scientists have uncovered the mysterious reason that thousands of octopuses travel to breed, brood, and die on a remote seamount two miles under the Pacific Ocean, in what is the largest known gathering of octopuses on Earth. Located about 80 miles west of Monterey, California, the so-called “Octopus Garden” provides an oasis of warm water that accelerates the growth of octopus embryos, providing a boost to their survival, reports a new study.

The Octopus Garden has astonished scientists and delighted the public since it was first discovered in 2018 by a U.S. National Oceanic and Atmospheric Administration (NOAA) expedition. Submersibles to the site have captured otherworldly visions of shimmering octopuses clustered along the slopes of a deep-sea volcano called Davidson Seamount.

Scientists solve mystery of why thousands of octopus migrate to deep-sea thermal springs - MBARI

An estimated 20,000 Muusoctopus robustus octopuses, also known as pearl octopuses, occupy this 823-acre site. While the Octopus Garden is clearly a breeding ground and nursery, scientists were initially puzzled about why such an unprecedented number of octopuses had selected this particular spot to reproduce.

Now, a team led by Jim Barry, senior scientist at the Monterey Bay Aquarium Research Institute (MBARI), has discovered that females take about 1.8 years to “brood” their young—meaning guard and nurture them until they hatch—at the Octopus Garden, which is far faster than expected for octopuses at these abyssal ocean depths.

The researchers concluded that “localized deep-sea heat sources may be essential to octopuses and other warm-tolerant species” and emphasized that “most of these unique and often cryptic habitats remain undiscovered and unexplored,” according to a study published on Wednesday in Science Advances.

The discovery of the Octopus Garden was “a total surprise,” said Barry in a call with Motherboard. “We’ve found octopus nurseries here and there, where you might find lots of octopus living on rock walls and laying their eggs, but this was way more than we'd ever seen before. Normally, ‘a lot’ would be hundreds. This is thousands and it's associated with these thermal springs, which is a new twist to the entire deep-sea ecology.”

Barry and his colleagues have spent the past few years exploring the Octopus Garden with the MBARI’s remotely operated vehicle (ROV) Doc Ricketts, which performed 14 dives to the site between 2019 and 2021. The team also left a timelapse camera in the garden that captured an image every 20 minutes from March to August 2022, opening a new window into the behavior of the octopuses over the long term.

The pearl octopuses that flock to the Octopus Garden are on a one-way trip, as the animals perish once they have reproduced. As a result, the site is like a bizarre crossroads between life and death where new hatchlings are born alongside the carcasses of their elders, all while thousands of females gradually exhaust their energy as they brood their young. Vulnerable eggs and hatchlings, along with the spent bodies of dead octopuses, fuel a broader ecosystem of predators and scavengers such as anemones, shrimp, fish, and snails.

The researchers counted 6,000 individual octopuses in the garden, and they estimate that the entire population is probably at least triple that number. All of the brooding females they encountered had specifically settled in warm spots around the site’s thermal springs, which reach temperatures of about 11°C (51°F) in contrast to the surrounding deep-sea temperatures of 1.6°C (35°F).

By amassing a wealth of observations, Barry and his colleagues were able to confirm that these relatively warm waters provide a metabolic boost that significantly shortens the brood period for pearl octopuses. Whereas some deep-sea octopuses take more than four years to produce offspring, hatchlings at the Octopus Garden emerge within two years. A faster brood period confers a reproductive advantage because it reduces the odds that eggs will be exposed to predation, illness, and other threats, according to the study.

“You can have a vast reproductive improvement by shortening that brood period,” Barry said. “We think that's really important for species that live in deep waters near very cold temperatures, so maybe that's the big advantage that we see for this octopus.”

The discovery of this enchanting octo-metropolis is yet another reminder of how little we know about the deep sea. Finding these ecosystems deep beneath the waves is the first step toward conserving them from a host of growing human-driven pressures, including climate change, plastic pollution, and deep-sea mining.

“We don't know enough about the deep sea yet,” Barry said. “We need to know more about it so that we can identify those areas that are really critical—not just for octopus necessarily, but for all the species involved in the ecosystem in the area. We can use that information to make more rational judgments about how we should manage these resources.”

“Not to mention, it just inspires us all because it enriches our lives to know that there are these really cool things in the world and we can appreciate them for their intrinsic value,” he concluded. “That’s part of it, too.”