The Metal Cap is an item 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 in Hazy Maze Cave.
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
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.