The cave is home to the most powerful Pokemon in the game with most being leveled in the 50s-70s. The cave even serves as the domain of the legendary Mewtwo, who is the most powerful, Wild Pokemon in the game.
The cave itself is a maze and you'll need your Pokemon to know almost every HM to be able to reach the end. The layout changes depending on the game your playing but one thing remains constant: You will be ceaselessy barraged by powerful Wild Pokemon until you either leave or lose. At the end of this labyrinth sits Mewtwo. You only have one chance to catch him so bring either many, many Ultra Balls or a single Master Ball.
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