Alterna

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

Description
Alterna is a large underground space located beneath The Crater in the Splatlands, built inside a massive geodesic dome lined with triangular liquid crystal panels depicting an artificial sky. Within this dome are six large islands—referred to as Sites—positioned around a large facility. Each Site features a multitude of Kettles, which are used by O.R.C.A to test Alterna residents' intellectual and physical prowess. Fuzzy Ooze covers large portions of the sites, blocking progress and hiding Loot Anchors or Locker decorations.

In Return of the Mammalians, 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.

History
Alterna is one of the few shelters created by humanity to have survived the disasters that ultimately led to the species' extinction. Built within a vast cavern created by a volcanic eruption, its inhabitants found sustenance in the marine life in the cavern's waters that had survived the apocalypse that devastated the surface.

Led by scientists in hopes of not repeating the mistakes that had led humanity to near-extinction, the inhabitants of Alterna established a sustainable ecosystem and created 3D-printed replicas of some of humanity's favorite cultural artifacts. Additionally, they created the Omniscient Recording Computer of Alterna to act as an autonomous record keeper of their collective knowledge.

25 years after Alterna was first settled, one scientist discovered a method to refine liquid crystals from the bodily fluids of squid. These crystals reacted to the thoughts and feelings of humans by projecting images of their desires, and were mass-produced and placed to line the walls and roof of Alterna, ultimately creating an artificial sky. However, when a rocket built by the next generation of humanity in order to return to the surface of the Earth performed a launch test, the energy from the boosters caused the crystals to overload, leading to a catastrophic cave-in that annihilated most of Alterna's population. What few survivors remained were left to wander until they too met their end.

The shattered fragments of liquid crystal dispersed within Alterna's bodies of water, drastically accelerating the evolution of the marine life within, imbuing them with echoes of the emotions they had absorbed—most notably including the humans' passive desire to see the Earth's surface once more. The marine life thus affected would eventually give rise to the majority of the life of the Mollusc Era—including the first Inklings and Octarians—who would discover the tunnels used by humans to escape the initial apocalypse and follow them to the surface.

The ruins of Alterna were abandoned for millennia, until the sole survivor of the Ark Polaris' crash landing located the subterranean facility using navigational equipment in the ship's wreckage. Mr. Grizz, as he would later come to be known, repaired some of Alterna's facilities and researched the remaining liquid crystals, eventually creating a terrifying new substance by compounding the crystals with his fur. Alterna became the headquarters for his plan to restore the planet to a mammalian paradise, with the still-intact rocket that had been Alterna's undoing to serve as the means of spreading the Fuzzy Ooze worldwide.

Shortly before Agent 3's arrival in Alterna, Agents 1 and 2 alongside their Captain set up a camp on Future Utopia Island.

Missions
Site 1: Future Utopia Island

Site 2: Cozy & Safe Factory

Site 3 - Cryogenic Hopetown

Site 4: Landfill Dreamland

Site 5: Eco-Forest Treehills

Site 6: Happiness Research Lab

Finale: Alterna Space Center

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

Trivia

 * Throughout Alterna various eerie audio clips of English-speaking voices that seem to be rocket launch announcements can be faintly heard. Totaling at seven clips, these announcements play at random, with each consecutive unlocking of a Site bringing a new clip into the "rotation".
 * These clips most likely emanate from the Alterna Space Center, as they are most easily heard when the camera is facing the facility.
 * During the infiltration of the Space Center during the finale, Announce 05 and 06 can be heard in the "intermission" segments between stages.
 * The location of Alterna on the official map of Inkadia and the Splatlands, combined with said map's resemblance to the real-life Tokyo Bay area, implies that Alterna may be built beneath what was once Mt. Fuji.
 * Dialog from talking with Agent 1 on Future Utopia Island and from Agent 2 when first visiting Cryogenic Hopetown implies that the snow in Alterna is not real, with the latter quote outright referring to it as "fake snow".