Submerged Castle

The Submerged Castle is a supposed event in Super Mario 64 DS where Princess Peach's Castle becomes completely submerged in water, and is tied to a method of unlocking Waluigi, as well as the Black Box.

How To Activate
The Submerged Castle event is only activated after several other steps are completed.

The first step is to collect all 150 stars and fly to the Black Box and ground pound it, which would spawn a vortex in front of the stained glass on Princess Peach's Castle that must be entered. It will lead to an altered version of the Mirror Room on the Second Floor. It will have a Goomba wearing a Luigi cap, and the Wario painting will be replaced with a Waluigi painting.

The second step is to defeat the Goomba, wear Luigi's cap, then use the Power Flowers to pass through the mirror and enter the Waluigi painting. Instead of going to Chief Chilly Challenge, the player will be taken to a clone of the character selection room. The Wario door will have an upside down L instead of a W, and the door to the Rec Room will disappear. Interacting with the door will say, "It's locked... Someone is screaming on the other side."

The third step is to enter the room where the Rec Room would be, which is now a black room. The player will need to platform up on top of a wall, and reach a chest that has an upside down L on the chest. Opening it will give the player Waluigi's key.

The fourth step is to unlock Waluigi's door, which will initiate a cutscene that ends with the player, playing as Waluigi, ending up at the Castle Grounds which will be completely submerged in water. This is the Submerged Castle event.

How To End
The Submerged Castle event is ended by swimming up to to the top of the castle, which will have a block containing a Power Flower. Collecting it usually gives Waluigi the ability to summon lightning, which must be used on the top of the castle to break a hole in the castle. The room accessed through this hole contains a switch that will drain the Castle Grounds, ending the Submerged Castle event and unlocking Waluigi permanently. Afterwards, the Black Box will disappear from the menu screen.