Cerulean City/Places of interest/Cerulean Cave

On the outskirts of Cerulean City lies Cerulean Cave, also known as the Unknown Dungeon. It is said to be home to "horribly strong ". The player can capture the level 70 in the Generation, , and  games. Only s who compete and win in the Pokémon League at Indigo Plateau may enter due to the strength of the Pokémon in the cave, although in Generation IV, the player must also defeat all of Kanto's Gym Leaders in order to gain access. Mewtwo lies at the end of the cave, the only one of its species in the game.

Another addition in is that Trainers are now required to further prove themselves before entering the cave by completing the quest on the Sevii Islands and restoring the Network Machine of the Pokémon Network Center to operational status. This stipulation may or may not have been added due to the addition of to the cave's selection of wild Pokémon, and the old set of requirements that would have allowed players to enter the cave before obtaining the National Pokédex.

In Generation II, the Dungeon has collapsed, but Mewtwo's Berserk Gene has been left behind. In HeartGold and SoulSilver, however, the cave is still open. The Pokégear map mentions that it once collapsed but was rebuilt, and Mewtwo can still be found inside.