Metal Cap

The Metal Cap appears in Super Mario 64. 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), they can no longer make very high or long jumps, and they sink faster in quick sand.

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

The Metal Cap's ! Switch can be found in the Metal Cavern, which can be reached by a portal in Hazy Maze Cave's underground lake area.