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The Cerulean Cave is filled with the strongest wild Pokémon. It's quite an interesting place to navigate, because it feels like you'll never reach where you want to go. After going through mazes several times, you'll find Mewtwo between two rocks.

[edit] Capturing Mewtwo

Mewtwo is a lv. 70 Psychic Pokémon; the strongest Pokémon in the game. Use the Master Ball, and hopefully, that wasn't used already. If it was, then prepare for the hardest Pokémon capture ever. Mewtwo is nearly impossible to catch. After catching Mewtwo, you should be proud to be the owner of the most powerful Pokémon!