Octo Valley

Octo Valley is the setting of the story mode in Splatoon.

Description
Octo Valley is the home base of the Octarians. It is comprised of many large underground domes, called kettles, and their interior walls are covered with screens that display images of the surface. To find the entrances to these kettles, you must find anomalies in your surroundings, as they are hidden, and shoot ink at them until they are fully visible. The air around these kettles will appear distorted, so they are not hard to spot. Using the WiiU gamepad, you can tap kettles you have found and super jump to them.

This is where the Octarians came to live after they lost the Great Turf War to the Inklings. The domes' falling into disrepair and losing electrical power drives the Octolings to invade the surface and steal the Zapfish.

Trivia

 * 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 seems to hint that Splatoon's events take place on Earth, long after humans have gone extinct.