Meta Crystal

From the Super Mario Wiki

Metallic crystals are located in the background.

Meta Crystal is a stage in the game Super Smash Bros.. It consists of one small platform hovering above a larger one. It is not unlockable, and was only used for one battle against Metal Mario in 1-Player mode. Even though the level is small, it's hard to knock Metal Mario off the stage, as he is so dense.

The background is made up of metallic crystals and a red wire fence. These objects could also be found in the Hazy Maze Cave and the Cavern of the Metal Cap in the game Super Mario 64.

This stage is the only one in the entire series not to be remade in another form or replaced in the sequels, unlike all others which were either updated (battlefield and Final Destination), or replaced with similar counterparts (series based levels), as the purpose of fighting metal characters were taken by Battlefield in Melee, and then by any random stage in Brawl. And because of this, it is also the only fighting stage to not be usable outside of Classic mode.