Cavern of the Metal Cap

Cavern of the Metal Cap is one of the three cap switch levels in Super Mario 64. Unlike the other two which can be accessed at Mushroom Castle, this level is accessed in Hazy Maze Cave. The player must direct Dorrie towards the elevated surface where a double door awaits and a metallic portal into the stage lives.

This level turns Mario into Metal Mario upon arrival, and holds the Green Cap Switch. It also has its own red coin mission like the other cap switch levels that encourages the use of Metal Mario's weight to walk underwater.

Geography
Cavern of the Metal Cap starts the player in a small area down a winding corridor with two Snufits. This corridor leads out into the main crux of the level, which is a long spanning cavern with rocky platforms to from, all above a flowing river. If the player falls into the water they will be swept away and out into the Castle Grounds. At the other end of the cavern tunnel is the elusive green cap switch.

There are eight red coins in this level, most of which can be found in the flowing river. To collect them, the player is advised to use the metal cap to keep themselves weighed down in the water instead of being swept away. Two red coins can also be found in a small alcove in the tunnel.

Break the Crystals
Break the Crystals is a supposed secret star that is said to exist in some people's copies of Super Mario 64. The few that recall this mission claim that the player had to use Metal Mario to smash the crystals with punches or kicks. Upon breaking them all, the player would be rewarded with a power star. When one player was asked about the crystals protruding from the walls that wouldn't be reachable, they said that they didn't remember any crystals on the walls.

The mission was likely created as a result of the Personalisation A.I. creating unique stars for the players. Destroying the crystals is not a far-fetched idea assuming that it uses a mechanism similar to that of the "5 secrets" object. Additionally, these players might've had more larger crystals to break than in the most circulated copy of the game.

The Metal Parasite
See: The Metal Parasite

Legacy
Meta Crystal, a stage from Super Smash Bros. on the Nintendo 64, is based off of Cavern of the Metal Cap and Hazy Maze Cave. It is where the player fights Metal Mario in the game's 1P mode.

Trivia

 * Cavern of the Metal Cap is one of the few areas in Super Mario 64 to have a known location around Peach's Castle. Other levels include Vanish Cap Under the Moat, Dire, Dire Docks and Wing Mario Over the Rainbow.
 * Since Cavern of the Metal Cap is linked to Hazy Maze Cave, it can be assumed that Hazy Maze Cave must be located near or around the castle since Cavern of the Metal Cap is situated behind the castle waterfall.