The Secret Under the Moat

Vanish Cap Under the Moat is a secret course in Super Mario 64 that contains the Vanish Cap's ! Switch. In order to access the course, Mario must drain the water in the moat of Princess Peach's Castle by pounding two certain pillars in the basement, then go outside to the newly-exposed door. After doing so, there will be a hole in the drained moat by the waterfall that will warp him to the course.

In Super Mario 64 DS, the course was retitled The Secret Under the Moat, and the blue ! Switch was removed. Also, Black Bricks are covering the entrance to the course, requiring only Wario's strength to enter. If he falls out of the course, he'll have to break the bricks again to enter.

Layout
When Mario first enters the course, he will be atop a large slide, with several ledges located scattered throughout. Upon entering, Mario will also be Vanish Mario, though nothing happens to Wario in the DS remake. At the bottom of the slide is a path consisting of various moving and teetering platforms that lead to the ! Switch. There are several Amps and Kuromame on the path. Once Mario pushes the ! Switch, all the blue ! Blocks in the game will become solid, making the Vanish Cap usable. If Mario falls, he won't lose a life, but will come out of a gaping hole near the waterfall.

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

8 Red Coins
There are eight Red Coins in the course. The first four are located on the slide's ledges at the start, while the other four can be found along the path to the ! Switch. The Star Marker is inside a cage next to the ! Switch, so after collecting the final Red Coin and pushing the switch, Mario will have to don a Vanish Cap to pass through the cage and nab the Power Star.

In the DS version, by collecting Luigi's Cap, Wario can use the Power Flower at the end of the course in lieu of a Vanish Cap and go through the cage to collect the Power Star.

Switch Star (DS only)
In the DS version is a new area across the chasm at the bottom of the slide. By turning into Mario, Wario can use a Power Flower to float over to the new area. By activating the Star Switch on the platform, the Power Star will appear in the Star Sphere above. Wario can reach it by either floating upwards to it or by using Wall Jumps.