Kettle

Kettles are story mode mechanics in the SS undefined. They are used to enter and exit Octarian domes in Octo Valley, Octo Canyon and The Crater as well as missions in  Alterna.

Splatoon and Splatoon 2
Kettles are devices that appear similar to tea kettles. Most of the kettles are silver—appearing to be made out of possibly aluminum or stainless steel. Boss Kettles appear to be stoneware or possibly pewter. In Octo Valley and Octo Canyon, kettles start off invisible, requiring Inklings to shoot them with ink to reveal them. Once revealed, they can be Super Jumped to at any time. Kettles are covered in a grate, meaning Inklings can only enter them in squid form via. "We Octarians dwell in a world deep underground. The individual caves in which we live are connected by a network of transport devices known as kettles. Wow... You learn something new every day!"

- Sunken Scroll 2

Splatoon 3
The kettles in Return of the Mammalians have a less round appearance compared to the ones in previous games. Along with silver kettles, there are also gold kettles. The gold kettles lead to missions that do not cost any Power Eggs, where the Hero Gear can be used. Agent 3 also always uses bright yellow ink in these missions. The silver ones lead to missions where other weapons are used, these also cost Power Eggs to enter. The Boss Kettles in Alterna appear to be cast iron. In, kettles no longer start off invisible, but must still be shot with ink to be opened. Entering a Kettle also now requires holding for a short amount of time.

Trivia

 * Because the events of Splatoon take place long after the extinction of humanity, relics of human history are scattered around Inkopolis. Kettles are likely based on tea kettles used by humans thousands of years in the past.
 * After a kettle's mission has been completed for the first time in Splatoon, stickers with squid designs in Agent 3's Ink color will appear on it.
 * When stood near, kettles will play the music that is played in the respective mission.