Octopuses have been dubbed one of the most mysterious and intelligent creatures in the sea. As would befit an animal that inspired epic monsters like the Kraken and Ursula the Sea Witch, it’s no surprise that research into their behavior only unfurls greater enigmas about the ancient cephalopod than it does provide answers, as illustrated in a study published in Deep Sea Research Part 1. 

Two deep-sea expeditions reveal a giant group of octopuses (no, it's not "octopi") and their eggs in a place where they shouldn’t be able to survive; 100 miles west of Costa Rica and over 3 kilometers (2 miles) underwater.

The Dorado Outcrop is a rocky patch of volcanic seafloor where hydrothermal fluids seep through crevices. Researchers were planning to take geological samples here when instead they found dozens of octopuses huddled around openings in the seafloor.

With “enormous eyes” and pink “dinner-plate-sized” bodies, the more than 600 octopods (admittedly some could have been double-counted) are believed to belong to an unknown species of Muusoctopus. 

"When I first saw the photos, I thought, 'They shouldn't be there! Not that deep and not that many of them!'" said zoologist and co-author Janet Voight in a statement. 

In the more than 200 hours of footage, a total of 11 egg clutches and 186 eggs confirmed the octopuses were indeed there, but not whether their endeavor was fruitful. Close examination revealed no embryonic development in the eggs.

Perhaps odder than their clustered behavior – Muusoctopus are normally loners – is why they would choose such a harsh environment to reproduce. Deep-sea octopuses are stenothermic, meaning they only survive in a narrow temperature range. Near these vents, temperatures range from 2.5-12.3°C (36.5-54°F), which likely speeds up egg development while increasing the octopuses’ metabolic rate and demand for oxygen. Together, elevated temperatures and low oxygen levels create a deadly environment.

Their choice is probably an accident. The octopuses probably attach their eggs to rock when the hydrothermal vents aren’t spewing out fluid, which can take place in as little as six days. Once attached, it's unlikely the new moms would want to leave their new nesting ground. Octopus eggs can take as long as 53 days to develop, and female octopuses only produce one clutch in their lifetime before dying shortly after. 

It's not all bad news for this rare group of octopuses. The unlucky octo-moms probably represent a much bigger population not yet seen.

"Odds are that this outcrop has hollow areas where other females nurture their eggs to hatching. They're analogous to boomers who have all the good jobs, while the millennials wait, seeking one little piece of cool rock,” said Voight.

According to the researchers, there are tens of millions more outcrops globally that more than likely have seafloor features, conduits, and networks similar to Dorado – a prime place for octopod egg brooding.