Scientists have just made an incredible discovery of an underwater “octopus city” that they are dubbing Octlantis, and they’ve observed some fascinating behavior from the octopi that live there. The octopuses have been seen communicating with each other by posturing, changing their colors, or chasing each others, according to new research from the University of Illinois.

Octlantis was found in shallow water off the east coast of Australia, and a total of 15 individuals were seen hanging out down there. It’s the second time scientists have found such a settlement in the area, indicating that octopuses may not be the loners everyone thinks they are. The first site was found in Jervis Bay in 2009, and the new site is just a few hundred meters away.

The site is about 10 to 15 meters under the surface of the water, and it is about 18 meters long and four meters wide. It includes a few patches of rock and shell beds, with 23 octopus dens, some occupied and some not.

“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,” said Stephanie Chancellor, a Ph.D. student in biological sciences at the University of Illinois at Chicago and an author on the paper. “In addition to the rock outcroppings, octopuses who had been inhabiting the area had built up piles of shells left over from creatures they ate, most notably clams and scallops. These shell piles, or middens, were further sculpted to create dens, making these octopuses true environmental engineers.”