Cavern of the Metal Cap (called Metal Cap Switch Course in the message that appears upon first entering the course) or Metal Cap Cavern is a cave in Super Mario 64 that contains the Metal Cap's ! Switch. The entrance is located within the Hazy Maze Cave course behind a set of doubled doors in the underground lake area. This 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, 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 in the later courses. The river has a strong water current and is moving very fast. If Mario falls in the water and does not get out in time, he will be lead to the waterfall outside Princess Peach's Castle, but will not lose a life. Directly before the exit through the waterfall is a ! Block containing a 1-Up Mushroom as well which can only be obtained if Mario wears a Metal Cap. The only enemies that appear in the N64 version are Snufits.
The area was retitled Behind the Waterfall in Super Mario 64 DS 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. The game also has a Bob-omb Buddy and a Goomba (which wears Wario's Cap if the player is not playing as him). Luigi can run on water without being swept by the water current.
There are two Power Stars to collect in this area, with one being exclusive to the DS version.
Star 1: 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 (or Metal Wario in the DS version). Once every Red Coin is collected, the Power Star will appear at the Star Marker (surrounded by a circle of Yellow Coins) underwater near the waterfall at the back of the cavern. It is possible to collect all the Red Coins and Star without becoming metal, though, it is a bit harder.
Star 2: Balloon Mario (DS only)
By using Mario, the player must hit the ? Block by the other side of the waterfall to obtain a Power Flower. By floating upwards from the spot, they can find a Power Star up above a triangular row of Coins.