From the Super Mario Wiki
Mario standing near a Metal Box
, as seen in Super Mario 64
The Metal Box is a green ! Block appearing in Super Mario 64. By breaking it, Mario is able to get the Metal Cap. The switch to turn on the Metal Boxes is located in the Cavern of the Metal Cap, a secret area inside Hazy Maze Cave.
The Metal Box also appears as an item in Super Smash Bros. Melee, and returns in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS, and is later going to appear in Super Smash Bros. for Wii U. When used, characters become coated in Metal for a short period of time, but also become much heavier, which means it is harder to KO them by blasting them away, although they fall much faster, making it harder to get back on the stage. Also, the voice of the character who is in use with the Metal Box is not heard. In Super Mario 64, however, Mario's voice is heard. Underwater, his voice is not heard unless he is falling from a great height. Additionally, a trophy of the Metal Box appears.
In Super Smash Bros. Brawl, Giga Bowser and Wario-Man are unaffected by Metal Boxes.
Super Smash Bros. Melee
| Metal Box
|| Super Mario 64
| First spotted in Super Mario 64, the Metal Box holds the power to turn your character briefly into heavy metal. Your mass is exponentially greater while metallic, making it difficult for others to throw you. Be careful though: not only will you move a bit slower, but you'll also fall much more quickly than you ordinarily would.
Super Smash Bros. Brawl
| Metal Box
|| N64 Super Mario 64
|| A green box that transforms whoever attacks it into metal. In metal form, you're a lot heavier than normal, so middling and even strong attacks don't move you too much, and standard attacks won't even make you flinch. The time you spend in metal form decreases with the damage you take. In Super Mario 64, the weight of the metal form allowed you to walk underwater.
Super Smash Bros. for Nintendo 3DS
||This green box will make you the ultimate defensive character, turning you into solid metal for a short time. You won't flinch from enemy attacks, even in midair,and it is much harder to launch you. On the other hand, you fall like a brick. Getting hit will reduce the duration of this item.
Name in other languages
||Bloque de Metal
||Boîte de Métal
||Green ! Box