Meta Crystal

Metal Cavern, also known as Meta Crystal, is an unlockable stage in Super Smash Flash 2 coming from the original Super Smash Bros. In Classic mode, the player will fight either a random metal character (on Normal difficulty or lower) or the Metal Mario Bros. (on Hard difficulty or higher). It can be unlocked by clearing Classic mode on Hard or higher without using any continues.

Layout
Metal Cavern is a relatively simple stage with an uneven main platform and a single, off-center platform resting above. It is the smallest stage in the game in terms of floor area, if one does not count Central Highway after being affected by its hazard.

Music

 * The main music track is titled Metal Cavern, an arrangement of the Metal Mario theme from the original Super Smash Bros.
 * The alternate music track is titled Metal Bros., an arrangement of the Metal character battle theme from Super Smash Bros. Melee.

Tournament Legality
Metal Cavern, as a legal stage, has a contested history.

In Project M, the stage is currently banned due to its tiny, uneven layout providing advantages to larger, stronger characters such as Bowser over smaller, faster characters that rely on having a wide space to move such as Pikachu. However, beforehand, it was a common counterpick and is still a popular choice for exhibition matches.

It has not yet been analyzed for Super Smash Flash 2 tournament legality, but is believed to at least be prepared for its viability to be put to trial in the early metagame.

Origin
Metal Cavern is a stage that only appeared in the original Super Smash Bros. On it, the player battled Metal Mario as a mini-boss before advancing to the Fighting Polygon Team and, finally, Master Hand.

The stage's design is very loosely based on the Cavern of the Metal Cap within Hazy Maze Cave in Super Mario 64, where Metal Mario is encountered for the first time as a power-up.

Meta Crystal, as it was referred to in the original game's files, could not be unlocked for players to use and could only be played on with the use of a GameShark or other cheating device. However, due to its popularity despite that, Project M included it as a standard playable stage, renaming it to Metal Cavern and giving it a new design, albeit with the original N64 graphics still available as an alternate option.

In Super Smash Bros., its Blast lines were massive, especially compared to the size of the stage itself, to enable the player to recover easier against the very resilient Metal Mario. However, in Project M, its blast lines were greatly reduced for the sake of ordinary play, which Super Smash Flash 2 uses instead.