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 river that will warp him to the course.

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 transparent, though it won't be of much use on the slide. 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 on the path, as well as fire traps, but they aren't a major threat. 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.

In Super Mario 64 DS, the course was retitled The Secret Under the Moat, and the blue ! Switch was removed. Also, a black Brick Block has covered the entrance to the course, requiring only Wario's strength to enter. If he falls (the same thing happens in the original) he'll have to break the bricks again to enter. It is one of the only three levels in the remake that only one certain character can access; The other levels being Chief Chilly Challenge for Luigi and Bowser in the Sky for Mario.

8 Red Coins
There are eight Red Coins in the area. The first four must be gotten on the slide 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 cage, there is also a square hole that can cause Mario can fall through.

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)
Another addition to the DS version was a new area near the bottom of the slide that housed a second Power Star. By turning into Mario, Wario can use a Power Flower to float over to the new area. This star is activated by hitting a Star Switch, and can be reached by either floating upwards to it, or by using wall kicks.