At the end of last year, scientists discovered a small octopus city – dubbed Octlantis – a find that suggests members of the gloomy octopus species (Octopus tetricus) are perhaps not the isolated and solitary creatures we thought they were.

Octlantis features dens made out of piles of sand and shells, and is home to up to 15 of the cephalopods, according to marine biologists. They recorded 10 hours of video footage of the site, which lies 10 to 15 metres (33 to 49 feet) under the water and measures 18 by 4 metres (59 by 13 feet).

The international team of researchers saw the gloomy octopuses meeting up, living together, communicating with each other, chasing unwelcome octopuses away, and even evicting each other from dens – so it seems Octlantis can be quite a rough place to live.

"These behaviours are the product of natural selection, and may be remarkably similar to vertebrate complex social behaviour," lead researcher David Scheel, from Alaska Pacific University, told Ephrat Livni at Quartz.

"This suggests that when the right conditions occur, evolution may produce very similar outcomes in diverse groups of organisms."

The new octopus city lies in Jervis Bay on the coastline of eastern Australia, and is close to another similar site discovered in 2009 called Octopolis – where we've seen a kind of Octopus Fight Club take place in the past.

To add to the sense of lawlessness, the researchers also discovered the discarded shells of eaten prey scattered around the city, and sometimes used to form dens.

Both these sites suggest that Octopus tetricus octopuses aren't quite the loners they've always been portrayed as, but what we don't know yet is whether these small octopus cities are particularly common, or exactly how they get started.

Octopolis seems to be centred on an unidentified human-made object about 30 cm (11.8 inches) in length, but there's no obvious comparable object in Octlantis that creatures appear to have settled around.

Instead it might be jutting rocks that first attracted the octopuses to the area, according to the researchers.

"At both sites there were features that we think may have made the congregation possible – namely several seafloor rock outcroppings dotting an otherwise flat and featureless area," says one of the team, Stephanie Chancellor from the University of Illinois at Chicago.

Usually, octopuses only get together to mate before going their separate ways again, but more research needs to be done to understand why they might want to mix together in places such as Octlantis.

There's an abundance of food at the two sites but they're also attractive to predators, and from what the researchers have observed so far, Octlantis seems like a rather violent and aggressive place to live.

One possibility is that these types of octopus settlements have always been around, but we're only now getting the technology and tools to be able to monitor them.

What we do know is octopuses are some of the cutest and coolest creatures living underwater, and if they want to set up subterranean cities then that's fine with us.

"We still don't really know much about octopus behaviour," says Chancellor. "More research will be needed to determine what these actions might mean."

The research was published in Marine and Freshwater Behaviour and Physiology.

A version of this article was first published in September 2017.