The Underwater City is located far beneath the Ocean of Ooo. It first appeared in "Ocean of Fear," when Finn was trying to face his phobia of the ocean. Debris from the aftermath of the Mushroom War litters the ocean's floor, further confirming the setting of the series. A black abyss can be located near to this place, where Jake once attempted to get into in Ocean of Fear. The city might have been underwater for a very long period of time due the distance from the city to the shore. It is unknown whether the city was collapsed from the mainland, or the Land of Ooo itself has a much higher topographical condition.
Underwater City mainly consists of debris of metals and rocks. Damaged buildings and vehicle can also be seen in Underwater City. Vehicles can also be seen submerged into a layer of sand. Some crystalline objects can be found on the ground.