Alterna

Alterna is the main location of single-player campaign, Return of the Mammalians.

Description
Alterna is large, spherical area deep under the Splatlands, with six islands positioned around a central rocket launch pad. Each island features a multitude of Kettles, which are used by O.R.C.A (also known as the Omniscient Recording Computer of Alterna) to test new Alterna residents. It is implied that anything found on the islands are the remains of human structures and technology. Fuzzy Ooze appears around the area, blocking paths until it can be removed.

In the single-player mode, the New Squidbeak Splatoon, consisting of Callie, Marie, the Captain, Craig Cuttlefish, and Agent 3, are all present in the location. Other characters that make an appearance in Alterna are Shiver, Frye, Big Man, and Mr. Grizz.

Alterna is one of the few shelters created by humanity to have survived the disasters that ultimately led to its extinction. Built within the depths of a volcanic crater, its people constructed the AI system O.R.C.A to act as a keeper of their collective knowledge. The shelter is lined with large triangle panels, which are made with liquid crystals created from the body fluids of the squids that lived in the surrounding underground ocean. These crystals reacted to thoughts and feelings of humans by projecting images of their desires, creating the artificial sky seen in Alterna. However, the attempted launch of a rocket built in order to return to the surface of the Earth caused the crystals to overload, leading to a catastrophic cave-in that annihilated most of Alterna's population, leaving the rest to wander until they too met their end. The liquid crystals dispersed within Alterna's bodies of water, drastically accelerating the evolution of the marine life and imbuing them with echoes of the emotions they had absorbed. The marine life thus affected would eventually give rise to the first Inklings and Octarians, who would discover the tunnels used by humans to escape the disaster and follow them to the surface.

Quotes
"Squid Research Lab here reaching out for a little help! We've identified several mysterious structures in the space known as Alterna. They're not complete, so we can't identify them. And when science falls short, scientists turn to... Twitter? Can you help us with these?"

- @SplatoonNA on Twitter

"Several mysterious structures have been identified in the space known as Alterna. They look as if they’ve been modelled after something, but they're all incomplete. What could be their significance?"

- @NintendoEurope on Twitter