This is another glitch in Super Mario 64 DS. To do this glitch, first perform Under the Castle Grounds. You must have not drained the moat for this to work! Now, dive under the moat so that you do not end up in the normal area, and go behind the castle. If you turn around, you will see the back side of the door in the moat. Dive down and touch it. The screen will fade to white, as if you had entered the Wing Cap Course. Now, you will be inside the castle in the room with the two pillars, but you will be walking underwater! Now, go back through the door, and you will be under the moat! You will also not be losing air. You can go anywhere under water without swimming, as long as you do not resurface! You can also get into Dark World 3.0.
Why it works Edit
As you touch the back side of the door, the game recognizes you as having gone through the door. You have to touch the TOP of the water for you to start swimming. And as you enter through the side, the game does not recognize you as being underwater. As you go back outside, the smae thing happens.
This glitch was discovered by Uberninja1337, but he may not have been the first one.