Behind the Waterfall

The Cavern of the Metal Cap (called the Metal Cap Switch Course in the message that appears upon the player entering) or Metal Cap Cavern is a cave in Super Mario 64 that contains the Metal Cap Switch. The entrance is located in the Hazy Maze Cave, behind a set of double doors in the Underground Lake area. It is the only level to be entered inside another level in the N64 version, and one of the only two levels to be entered inside another level in the DS version, the other being Big Boo Battle in Big Boo's Haunt. In order to reach it, the player must traverse to the underwater cavern in the Hazy Maze Cave. The player then has to get on top of Dorrie's head and then steer him over to the small patch of ground outside the double doors. Upon entering the double doors, the player will have to go through a short hallway, at the end of which is another pool of liquid metal, much like the one that is used to enter the Hazy Maze Cave in the first place. Falling into it will transport the player to the Cavern of the Metal Cap.

In Super Mario 64 DS, the area was retitled Behind the Waterfall, and the Cap Switch was removed. However, a second Power Star was added, which can be reached only by Mario using a Power Flower. Signs, however, continue to refer to the area as the Metal Cave or Metal Cavern, which is a holdover from the original version.

Layout
The cave is decorated with large glittering crystals and has a stream of water running through the middle. In the original game, upon entering the cave, Mario is automatically turned into Metal Mario and is therefore invulnerable to the stream's current. The water is moving very fast and can sweep Mario out the cave's one-way exit (which will lead him to the waterfall outside the Mushroom Castle) if he is not careful. Directly before the exit through the waterfall is a yellow block containing a 1-UP Mushroom as well, which can be obtained only if Mario gets a Metal Cap. The only enemies that appear are Snufits, and a Goomba in the DS version. The DS version also has a Bob-omb Buddy. If the player presses the Metal Cap Switch, all of the green blocks in the game will activate, granting access to the Metal Cap in later courses.

If the player gets swept out of the course, they will be brought to the Castle Grounds with no life lost.

Missions
There are two Power Stars to collect in this area, with one being exclusive to the DS version.

8 Red Coins
There are eight Red Coins to be collected here, and several of them are just below a huge waterfall leading into the stream. The only way to reach them is by becoming Metal Mario (Metal Wario in the remake). Once every Red Coin is collected, the Power Star will appear at the Star Marker underwater near the waterfall at the back end of the cavern, surrounded by a circle of Yellow Coins.

Balloon Mario (DS only)
By using Mario, the player must hit the red block by the waterfall to obtain a Power Flower. By floating upwards from that spot, they can find a Power Star above a triangular set of coins.