The Submerged Castle

Not to be confused with the Sunken Castle, a similar anomaly in the original Super Mario 64. The Submerged Castle is an anomalous event in Super Mario 64 DS. When the event occurs, the entire Castle Grounds will be flooded with water. This is usually involved in unlocking Waluigi or some other chain of anomalous events. You can tell this event will occur in your copy when after draining the moat and collecting every star in Vanish Cap Under the Moat, the moat will fill back up with water.

Description
The Submerged Castle event most commonly occurs when Waluigi has lightning powers in the player's personalized copy, but this can occur if Waluigi is involved with some other abilities he can have, such as the Magic Gloves, Bob-ombs, or warp-speed ability, the latter of which to get through the water. These are typically required for the process to end the event, which must be done as the castle cannot be entered during this anomaly's time in the save file.

How to Trigger
The event itself is usually triggered with the following method of unlocking Waluigi, known as the Black Box method: The player will need to use Mario's Wing Cap in the Castle Grounds to fly into the Black Box and break it, usually just by making contact or by somehow ground pounding it. This will open a portal in front of the stained glass, or the stained glass itself. The player will then need to shoot into it to reach an area behind the stained glass. This area can reportedly contain a painting of Waluigi that would lead to a variant of the character selection room with a door with an upside-down L, indicating Waluigi is inside. The player can then enter the door that would lead to the Rec Room and enter an area that has been described as a pitch-black maze. This area is the most inconsistent part of this method, and may lead elsewhere, such as Rabbit Meadow or the Piranha Pit. At the end, however, Waluigi's key will be present. Upon opening the door, the player will become Waluigi and enter the Submerged Castle. Note that any transition from area to the next can lead to the Submerged Castle depending on the report.

Event Guide
Once the event occurs, the way to end it is relatively consistent unless the method to trigger it is unrelated, but it will usually still require something similar to this process. The player has to swim up to the top of the castle and find a ! Box that contains a Power Flower. This will usually trigger Waluigi's required ability if he is required, but if he is not being used, the Power Flower ability can still reportedly be useful, especially if the player is Wario. The player then has to use this ability to break a hole into the roof of the castle or the central tower, revealing a switch that will drain the castle, allowing the player to continue with the game as normal, sometimes unlocking something such as Waluigi if he was not already unlocked or the 151st Star.

Theorizing
While this event may seem bizarre and seemingly meaningless, it is worth noting it's connections to the original game. Not only can the Black Box reportedly be found in the original Super Mario 64, so could an area behind the stained glass, and the Sunken Castle. The most interesting connection is that one of the images of the Sunken Castle contains a stained glass that portrays a yellow diamond, potentially referencing the lightning ability of Waluigi that is commonly associated with the Submerged Castle. What this means is unclear, but this appears to suggest that Super Mario 64 DS' Personalization A.I. is replicating phenomena from the original game, or that this unused event was made by the developers playing a personalized copy of Super Mario 64 to base the remake off of.