Behind the Waterfall

Cavern of the Metal Cap is a cave in Super Mario 64 that contains the Metal Cap's ! Switch. The entrance is located in Hazy Maze Cave, behind a set of double doors in the underground lake area.

In order to reach it, the player must follow almost the exact same procedure as in the level "Swimming Beast In The Cavern" in Hazy Maze Cave. Mario must first get on top of Dorrie's head, and then steer it over to the small patch of ground outside the double doors. Upon entering the double doors, Mario will be confronted with a short hallway, at the end of which is another pool (much like the one that is used to enter Hazy Maze Cave in the first place) which will transport Mario to the Cavern of the Metal Cap when entered.

The cave is decorated with large glittering crystals and has a stream running through the middle. 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 Peach's Castle) if he is not careful. Upon entering the cave, Mario is automatically turned into Metal Mario and is therefore invulnerable to the stream's current. If the player hurries, they can quickly defeat the few Snufits guarding the Green ! Switch and press it. All the green ! Blocks in the game will then activate, granting access to the Metal Cap in the future.

In Super Mario 64 DS, the area was retitled Behind the Waterfall, and the Green ! Switch was removed. However, a second Power Star was added, which can only be reached by Mario via a Power Flower.

Levels
There are only two missions seen in this world.

Star 1
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/Wario. Once every Red Coin is collected, the Power Star will appear behind a small island near the waterfall at the other end of the cavern.

Star 2 (DS only)
Using Mario, the player must hit the ? Block by the waterfall to obtain a Power Flower. By floating upwards from that spot, they can find a Power Star up above.