When the Octarians lost the Great Turf War, they were left to reside in a few locations like Octo Valley and Octo Canyon. These areas consisted of several underground domes that were filled with ancient human structures. The Octarians used these ancient artifacts to create most of the obstacles and mechanics seen in the games.
During the events of Splatoon, the domes in Octo Valley were deteriorating due to a loss of energy causing most to be uninhabitable. DJ Octavio, the Octarians' leader, then decided to steal the Great Zapfish from Inkopolis for his own Octoweapon and used all the other Zapfish to power the domes.
In Splatoon 2, Callie was kidnapped and brainwashed by Octavio. According to Sunken Scroll 23, Callie was put in charge of redecorating the domes to look more upbeat. This is why several banners, stickers, and balloons can be seen in all of the domes. It is also implied that these are the domes that the Octarians moved to after the constant deterioration of the Octo Valley domes.
The Octarian domes are caves with several large television screens broadcasting the outside sky. Each dome includes a giant lamp that brings light into the entire cavern. They also contain several other floating objects like boulders, buildings, and railways.
While the domes in Octo Valley only serve as battlegrounds for protecting the Zapfish, the ones in Octo Canyon are also shown to have many other varieties like cities and resorts. The "city domes" have several suction cup-like buildings scattered along the bottom of the area. The sky panels here also have a hexagonal shape and are neatly placed, unlike the other domes. The "resort domes" are filled with water at the bottom of the cave along with a few islands along the sides. The sky panels also broadcast more tropical scenery. The third type of dome also appears to have a more basic cave design, though it does appear to have several roads going across the bottom and top of the dome. Like the resort type, they also are filled with water at the bottom of the cavern.
The final fight with DJ Octavio in Octo Canyon also takes place in his own "concert dome". This dome has several seats along the walls of the cave for Octarians to view the battle. There are also a few additional panels that broadcast close-ups of Callie and Octavio during the battle. This dome also seems to be filled with a lake of purple ink instead of water.
- The official Japanese Splatoon Twitter account stated that Octo Valley "was made using the remains of an ancient civilization. It seems to be using very familiar things... or is it just my imagination?" This was an early hint that Splatoon's events take place on Earth, long after humans have gone extinct.
- Boss battles feature a quiet but startling mechanical ambience accompanied by screeches and moans that sound similar to Zapfish cries. This usually cannot be heard due to music and sound effects during gameplay but is easy to listen to before and after battles. The source of these eerie noises is unknown. When asked about the noises, Splatoon director Tsubasa Sakaguchi responded "It's the first time I've heard this! I was the one who was in charge of the mechanics and the effects for the final boss. Towards the end, though, I was running out of time so I was asking the staff around me to do things for me - like the staff credits and many other things. Maybe it's a curse from those other staff. The final boss was one of the last things that we were working on. We were all high on adrenaline. Maybe there's some element of that in there, of that energy in there. Once I go back to Japan, I'll play it and have a look."
- The Shifty Stations (aside from MC.Princess Diaries) appear to be built in Octarian domes or at least share many similarities with them, such as having floating objects and octopus-shaped rocks. This is due to them being built by Marina.
Concept art of locations in Octo Valley and the domes.
Several Octarians inside a dome.
Two Shielded Octotroopers in a dome.
A Twintacle Octotrooper in a dome.
Two Octoballs in a dome.
An Octosniper in a dome.
An Octostamp in a dome.
An Octoling in a dome.
Agent 3 in Flooder Junkyard.