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Meta Crystal (メタ・クリスタル), also known as Metal Mario's Stage, Metal Cavern, and a variety of other names, is the stage on which Metal Mario is fought in the 1P Game of Super Smash Bros. This stage can be accessed only in 1P Game or through cheat codes, hacks, or other devices.
The stage is made up of one small platform and a base platform. The bottom one is slightly raised to the right and lower to the left. Above the middle of the bottom platform is a floating platform. There is a metal path in the middle of the larger platform on the stage.
The stage is quite small, probably to counter how difficult it is to hit Metal Mario offstage.
This battlefield is most likely based on Super Mario 64's Cavern of the Metal Cap, considering that both contain Metal Mario and various crystals. The platform and wire walls match those of Hazy Maze Cave, which is the only way to access Metal Cavern.