In the earth only one ocean had remained; its name had changed many times over time, but in the current era they called it the Aram Ocean, and at its depth the military had made a dungeon called Mariana Prison. 14000 meters under the surface, under the water, only highly advanced submarines could go there without being crushed by the pressure of the water.
More famous than the prison itself was its warden, ex-general Kraken. Before his retirement, Earth had no place to imprison the most dangerous criminals, but after he held the Mariana prison, he moved there to play his role as its warden. Thousands of years had passed, and the title of Kraken was passed through his family as they kept the world safe from the prisoners outside.
There were too many unknowns in there, with a whole family with water magic, deep in the ocean; no one ever dared to attack the prison from the outside.
