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.
Underwater City mainly consists of debris of metals and rocks. Damaged buildings and vehicle can also be seen in Underwater City. A black abyss can be found near to this place, where Jake once attempted to get into in Ocean of Fear.