In Kingdom Hearts, Atlantica is an underwater world that you must see included in your quest in order to get to the Hollow Bastion and rescue your buddy Riku. As it has, like many of the places you visit in the sport, been taken over from the Heartless, who are the bad and dark enemies of the game you have to see this world. You must first find King Triton (the ruler of Atlantica) who'll warn you of Ursula before delivering you on your own mission.