The Metal Cap is one of the three Caps in Super Mario 64, which has the power to change Mario into Metal Mario. Mario activates them by hitting a Green ! Switch in Cavern of the Metal Cap, a secret course found within Hazy Maze Cave. Once he steps on the Green ! Switch, they will appear on every Green Block that he finds.
While in this state Mario gains temporary invulnerability and becomes much heavier, giving the player many advantages and a few disadvantages. For instance, Mario is immune to the toxic fog, but he will sink faster in quicksand. He can also sink in water, even in fast moving streams.
Super Mario 64/Super Mario 64 DS
The Metal Cap first appears as a power-up in Super Mario 64. In this game, once Mario collects a Metal Mario Cap, his body transforms into metal, turning him into Metal Mario.
The Metal Cap is represented as the box instead in an item called Metal Box. This started in Melee but continued to the most recent entry. It turns the character heavier but, less likely to flinch and harder to send flying. It is also a option to turn all players into Metal forms in Special Smash and is a modifier in the single-player modes.