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The Unknown Dungeon (Japanese: ななしの洞窟Nameless Cave) is a very small cave in southern Kalos that can be accessed from Pokémon Village. A sole Mewtwo resides in the center with either the Mewtwonite XX or Mewtwonite YY. The Unknown Dungeon is only accessible to Trainers who enter the Hall of Fame. Surf is needed to access the cave.
Its purpose as a place to catch Mewtwo is a direct reference to Cerulean Cave in Kanto. It also shares its name with the alternative moniker for Cerulean Cave used in Japanese releases of the Generation I games.
Geography
The Unknown Dungeon is a tiny cave whose entrance is found in the side of a mountain. The cave is barren in terms of terrain aside from the occasional water drop and the few rocks that dot the floor. Mewtwo awaits the player in the center of the cave, facing away from the entrance.
A Mewtwo appears here on the first visit. If defeated or run from, it respawns after entering the Hall of Fame. It is coded to never be Shiny. After being caught, it will leave behind its Mewtwonite XX or Mewtwonite YY, depending on the game.
The Unknown Dungeon appeared briefly in Malamar Traps, where Mewtwo was shown residing inside the cave, waiting to make its move against Team Flare, who had taken over Pokémon Village and were capturing local wild Pokémon to be used as a power source for their second attempt to use the ultimate weapon.
