Unknown Dungeon

The Unknown dungeon is a cave in Pokemon Red, Blue, Yellow, and Green. Access to it is denied until you defeat the Elite Four and the champion. In it, Mewtwo (a monster cloned from mew) resides. You only have one chance to capture it. Mewtwo is indisputably the most powerful pokémon in the entire game obtainable without using cheats. It is a Phychic type, and is Level 70 when you fight it. A popular technique for catching it is to use the Master Ball you are given, as it is the most difficult pokémon to catch otherwise.