On a rocky outcrop almost 2 miles beneath the surface of the Pacific Ocean off Costa Rica, researchers have documented an active octopus nursery. It may be the third known example of a brooding site where huge numbers of the creatures cluster together.

During a three-week expedition this month, scientists on board Schmidt Ocean Institute’s Falkor research ship saw hundreds of octopuses and watched their babies hatch at the soccer field-size Dorado Outcrop. Located 2,800 meters (1.7 miles) deep in the lower reaches of the so-called twilight zone, it was one of six underwater mountains surveyed by the vessel’s underwater robot, ROV SuBastian.

“We could see that some of them were ready to pop — like the octopus just came out,” said geomicrobiologist Beth Orcutt, a senior research scientist at the Maine-based Bigelow Laboratory for Ocean Sciences, who led the expedition along with Jorge Cortés of the University of Costa Rica. “It was a really exciting moment because we weren’t expecting it.” Cortés is a researcher at the university’s Center for Research in Marine Sciences and Limnology.

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The discovery of an active community of octopus moms and babies solved a mystery that had perplexed scientists.

A subsea vehicle first spotted a large group of octopuses on the Dorado Outcrop in 2013 during an expedition to understand the area’s geochemistry.

It was the first time that octopuses, typically solitary creatures that like cooler waters, had been spotted clustered in such numbers in this way on a low-temperature hydrothermal vent — an opening in the seafloor where warm, chemical-rich fluids flow out.

Scientists observed that the octopuses appeared stressed and could not see any developing embryos in the 186 eggs they counted in still images and video footage. The research team thought conditions were too inhospitable to make it a great place to start a family.

“I was part of the team that went to the site (10 years ago). We just went to that one outcrop to look for evidence of low-temperature vents and discovered that there were those octopuses, but they didn’t seem to be successfully brooding,” Orcutt said.

“So the goal was to go back to Dorado and figure out what’s going on, and we discovered it is an active nursery. We saw babies being born.“

The researchers also found one other smaller octopus nursery on a low-temperature hydrothermal vent on an as-yet-unnamed seamount.

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The discoveries add to evidence that some species of deep-sea octopus may seek out low-temperature hydrothermal vents for brooding their eggs. The warmer fluids may provide an advantage to egg development, even if the lower oxygen would make it harder to breathe. Octopuses are famously self-sacrificial mothers — after laying a clutch of eggs, they quit eating and waste away, typically dying by the time the eggs hatch.

“We don’t know why they are attracted to this place. Perhaps they just stumble upon it, perhaps they seek it out, maybe for the warmth, maybe for the bare rock, maybe for some reason beyond our comprehension,” said octopus specialist Janet Voight, an associate curator of invertebrate zoology at Chicago’s Field Museum who was part of the latest expedition. She was one of three authors of a 2018 paper analyzing the initial footage taken in 2013.