On the outskirts of Cerulean City lies Cerulean Cave, also known as the Unknown Dungeon. It is said to be home to "horribly strong Pokémon". The player can capture Mewtwo at Level 70 in the Generation I, III, and IV games. 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.
♥ You will not be allowed to enter cave until you have all eight kanto badges ♥
Mewtwo Level 70:
Item Attached: Leftovers
I understand with love comes pain, but why did I have to love so much?