Princess Peach's Castle/Internal Garden

The Internal Garden is a room reportedly found in Princess Peach's Castle in some personalized copies of Super Mario 64. It is most commonly found on the 4th Floor of the castle, though other reports claim it can appear elsewhere, such as the 3rd floor, Bowser's Domain, or even His Domain. It is commonly reported to have grass on the floor, water of some kind, the unused Dancing Flowers and trees, making it appear as if it was an outside area. It reportedly has a unique music track, most commonly described as being an acoustic version of the song from Dire, Dire Docks, though some claim it just has an outside ambient track.

Description
The Internal Garden is generally described as appearing similar to the Castle Courtyard or Castle Grounds, only being in an inside room instead. The ground will be dirt covered in grass, with a body of water somewhere, alongside flower patches, trees, and in some copies have the unused Dancing Flowers present. Most reports say it has windows around the walls or the ceiling and/or a slightly altered version of the first floor's wall texture, likely due to it most resembling the outside world.

Upon entering, there is reportedly a sign that welcomes the player inside, similarly to the Castle Garden area, and a character of some kind that can be interacted with. It is most commonly a Toad, though it reportedly can also be a Bob-omb Buddy, Koopa, or even the Wet-Dry World Girl, usually baring different designs from their regular counterparts, such as the Toad having a green vest in some reports. Some have reported that they can give the player a Power Star or a mission of some kind, such as clearing out enemies in the area if present or somewhere else such as the 4th floor, though these reports are relatively uncommon. The room has also reportedly held a statue of some kind, either the Toad Shrine, Eternal Star Statue or even a statue depicting a Power Moon, though this is rare to see.

The Internal Garden can reportedly lead to levels in some copies, typically grassy and sunny levels such as Bob-omb Village, Flower Fields and Piranha Valley. Some have claimed jumping out the windows leads somewhere such as the Moon Courtyard or Castle Fields, though these are not commonly mentioned. It is worth noting that all of these areas can also be found outside of the Internal Garden in other copies, leading some to suggest these reports were faked as the Internal Garden would likely have it's own levels instead of borrowing from elsewhere.

Positive Emotional Aura
Unlike Wet-Dry World, there is reportedly a Positive Emotional Aura found in the Internal Garden. This is generally believed to be caused by the contrast in feel from the 4th Floor where it is most commonly found, the calming music that is reported, and the area's design itself. Some have even claimed they didn't want to continue playing, and that they were "too scared to go out."

Calmed Music
Some have reported that upon approaching the door to the Internal Garden, the background music of the area they're in will get calmer and quieter than how it normally is. Many believe this was done by the developers of the game to transition the player into the Internal Garden due to how different it is to other areas. This is typically used to locate the Internal Garden in copies where it is harder to navigate the castle, and may contribute to the Positive Emotional Aura in some way.

Theorizing
It is believed that the Internal Garden is a "gift" to the player if they are having a hard time progressing by giving them a calm area to explore outside of the main game. This may explain why most reports say it does not have any levels and is simply an area that can be explored, and also usually appears after the player typically roams outside areas and the castle itself instead of actually playing levels. However, this also means the copy is heavily personalized due to the fact an entire subarea of the castle is added to accomplish this, meaning that similarly to the White-Eyed Chomp several use the Internal Garden as a marker for extremely personalized copies.