Cavern of the Metal Cap (called Metal Cap Switch Course in the message that appears upon first entering) or Metal Cap Cavern is a secret course in Super Mario 64 which houses eight Red Coins and the switch that activates the Metal Cap's ! Switch. Once the green switch is hit, the Metal Cap becomes available to use. The entrance is located within the Hazy Maze Cave course behind a set of doubled doors in the underground lake area. In order to reach it, Mario 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, you will have to go through a short hallway, at the end of of which is another pool or liquid metal, much like the one used to enter Hazy Maze Cave in the first place. Jumping into it will transform you to the area. When you first enter the doors, Mario will start as Metal Mario. Then he will have to defeat a couple of Shifts guarding it. After Mario hits the green switch, the Metal Cap can be used. Through here, Mario enters the Cavern of the Metal Cap. It is a small cave with only a Snifit, Amps, a Goomba in the DS version, and several coins. Inside the cavern is a fast flowing stream. If the player falls in the water and does not get out in time, the player will be transported to the waterfall outside the castle, and into the lake and moat.
In Super Mario 64 DS, the area is retitled Behind the Waterfall and and the green ! Switch was removed. However, a second Power Star was added, which can only be reached by Mario using a Power Flower. A sign, however, continues to refer to the cavern as the Metal Cavern.
There are two Power Stars to collect in this area, with one being exclusive to the DS version.
8 Red Coins
There are 8 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 DS version). Once every Red Coin is collected, the Power Star will appear at the Star Marker underwater near the waterfall at the back of the cavern.
Balloon Mario (DS only)
Using Mario, the player must hit the ? Block by the waterfall to obtain a Power Flower. By floating upwards from the spot, they can find a Power Star up above.
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 in another level in the DS version; the other being Big Boo Battle in Big Boo's Haunt.