Metal Cap

The Metal Cap is an accessory that makes its debut appearance in Super Mario 64. It is usually found in Metal Boxes. The Metal Cap can turn Mario into Metal Mario, which makes him invulnerable. The side effect of this temporary invulnerability is that the user becomes extremely heavy (with a side effect of extremely powerful Ground Pounds), can no longer make very high or long jumps, cannot swim, and will sink faster in quicksand. Fortunately, while in metal form, the player is also unaffected by toxic fog, and cannot drown.

If Mario were to get another power while the Metal Cap is in effect, they will combine, most notably on Dire Dire Docks, where a Metal Box is located directly beside a Exclamation Mark Block.

In Super Mario 64 DS, the Metal Cap does not appear. Instead, Wario becomes Metal Wario when he comes in contact with a Power Flower; but still retains the same properties as Metal Mario. As Wario couldn't use other Cap forms, it was not possible to get a Vanish Metal form in this game.

A similar item, the Metal Mushroom Orb works almost identically to the Metal Cap.

Trivia

 * A sign by the entrance to the Toxic Maze in the Hazy Maze Cave says, "Hidden inside the green block is the amazing Metal Cap. Wearing it, you won't catch fire or be hurt by enemy attacks. You don't even have to breathe while wearing it. The only problem: You can't swim in it."
 * In Super Smash Bros. Brawl, Mario can do a taunt where he spins around and removes his cap. Despite apparently removing his Metal Cap, this does not prematurely end his time in metal form if he does this while Metal Mario.