Wikia

The Metal Cap is an item which first appears in Super Mario 64. It is activated when one presses the green switch in the Cavern of the Metal Cap in Super Mario 64 or when one collects all 8 Red Coins in Super Mario 64 DS. It is usually found in a green box. When worn, the player is invulnerable (except for taking damage from falls) and can walk underwater. They may also walk on lava without danger, though will be slowed severely and have a reduced jump. Additionally, it will cause enemies damage on contact in a similar way to a punch. In Super Mario 64 DS, Wario gets the same powers the Metal Cap grants when he picks up a Power Flower.

The Metal Cap also appears in the Super Smash Bros. series, specifically Melee and Brawl.