The first time that divers discovered a "city" of octopuses off the coast of Australia, it seemed like a fluke. Octopuses are notoriously solitary animals. Divers found a small group of them in 2009 living together in burrows built around a piece of discarded metal, and they called it "Octopolis." At the time, scientists believed it was a rarity, perhaps caused by human meddling in the environment. But, in 2016, divers found another community of octopuses living in dens built from discarded shells. And this time there was no hunk of metal that had disturbed the natural environment. Researchers now suspect octopuses have been building group habitats for a long time.

Alaska Pacific University marine biologist David Scheel and his colleagues described the new discovery in the journal Marine and Freshwater Behavior and Physiology. They call it "Octlantis," and, over several months of observation, they determined that the settlement is made up of roughly 10 to 15 gloomy octopuses (yes, that is actually the common name for Octopus tetricus). Octlantis is hardly a metropolis, though—it's more like a tiny village of dens clustered around rocks, built up over several generations. These octopuses only live for about three years, so each generation is relatively short. But they leave behind mounds of discarded shells from their prey, as well as junk they've scavenged, like beer bottles and lead fishing lures. Over the years, octopuses pushed these mounds against the rocks, burrowed inside, and created dens next to each other.

Using video footage from divers and camera traps, the scientists were able to observe the social behavior of city slicker octopuses. They saw at least three pairs of neighbors mate, and there were also a few near-miss "attempted" matings. Mating must have been particularly strange for the octopuses in close quarters, because their reproductive systems are optimized for long-distance fertilization between the solitary animals. Typically, octopuses mate when the male shoots a package of sperm called a spermatophore at the female through the water. The barbed package burrows into her skin and releases the sperm. No touching is required.

The researchers also got a lot of footage of what can only be called octopus evictions. One octopus would reach into a burrow and pull its occupant out, then move in. In one case, the evicting octopus followed its victim to a new burrow and yanked it out again. Sometimes these evictions led to physical fights between two octopuses, which are dangerous for a number of reasons. Often sharks will hover around Octlantis, just waiting for the octopuses to emerge. Fighting octopuses are distracted and therefore perfect prey.

Building Octlantis might also put the octopuses in danger, since they have to come out of hiding to pile up shells and create burrows. According to the researchers, Octlantis residents also regularly engage in social behavior that humans have never witnessed between octopuses before. All these activities require them to leave their shelters and expose themselves to predators far more often than solitary octopuses do.

Given the dangers of octopus city life, what would make these animals want to live together? Possibly for the same reason that other species cluster into groups. "Congregations such as these probably occur wherever shelter is limited to small patches of habitat, and food is plentiful," Scheel told Quartz. The rocky outcropping and metal debris at the cores of Octlantis and Octopolis are rare examples of shelter-ready regions on a generally flat ocean floor. Plus, the shell mounds of the cities create perfect habitats for scallops and other octopus food. It's likely that octopuses gathered in these spots for the shelter and food, and their social behaviors evolved over time.

Scheel and his colleagues add that other species of octopus have been observed living in clusters of dens. Perhaps octopus cities are more common than we believed. It's only now that so many people are diving with cameras that we are seeing the full range of octopus behavior. We have much more to learn about these mysterious, intelligent animals who sometimes choose to live alone and sometimes choose to live with neighbors.