Cerulean Cave (Japanese: ハナダのどうくつ Hanada Cave), also known as the Unknown Dungeon (Japanese: ななしのどうくつ Nameless Cave) in Generation I, is a cave located in the northwest corner of Cerulean City in Kanto. The cave is home to a wide variety of high-level wild Pokémon. Only Trainers who compete and win in the Pokémon League at Indigo Plateau may enter, due to the potency of the Pokémon in the cave. At the end of the cave lies Mewtwo, the only one of its species in the game.
In Pokémon FireRed and LeafGreen, players must further prove themselves before entering the cave, which is done by completing the quest on Sevii Islands and restoring the Network Machine to operational status. More Pokémon can be found only by fishing, while wild Pokémon found just by walking around in the cave are reduced in number. Pokémon FireRed and LeafGreen also fixed the awkward Pokémon distribution from the Generation I games, which included several misplaced species such as Dodrio and Pokémon that are otherwise only obtainable by stone-induced evolution.
In Generation II, Cerulean Cave completely collapsed but the leftovers of Mewtwo's presence remain and are found in the lake near the cave, in the form of the item Berserk Gene.
In Pokémon HeartGold and SoulSilver, Cerulean Cave has been rebuilt and Mewtwo can once again be found inside. Like in previous generations, a NPC stands in the way until the player has obtained all eight Kanto Gym Badges (as opposed to defeating the Pokémon League at Indigo Plateau, the requirement in the Generation I games and Pokémon FireRed and LeafGreen).
The Unknown Dungeon in Kalos is a direct reference to Cerulean Cave, going as far as using the alternative moniker of Cerulean Cave from Generation I as its name.
Cerulean Cave is the main place where players can find Mewtwo, Mew, and Brock when they enter. There will be no Fusions there until the player hits level 50; then a call will come from Brock, saying that Fuse has attacked there. Then the player will have to complete the Mew Nano Mission.