Princess Peach's Castle/Internal Garden

The Internal Garden is a room found on the castle's Fourth Floor.

How to find
The door to the Garden is located at the far end of one of the halls. The player can understand if they're on the right track by listening carefully to the background, as the more one gets closer to the door, the quieter the music becomes.

Description
How the Internal Garden looks is different from copy to copy, but there are some constants. For example, there is always a small pond at the center. The walls are azure - likely to simulate the sky - and the floor has the same texture of grass, along with the same SFX. There are flowers at the edges of the room (this is where the unused flower texture can be found. The flowers are usually positioned northwest and northeast to the door, and their colors vary). Next to the door, there's a sign that reads:"'Welcome to the Internal Garden, where you can find peace of mind. no littering please!'"On one corner of the room, a Toad can be found. Of noting is its vest; instead of blue, it is green. If interacted with, they will say:"'Oh hello Mario! Sorry I didn't notice you, I was busy watering the flowers. Can I tell you something? Here it is: I love this job, it's super relaxing, but those stupid Boos keep comin' in and out and they pull pranks on me all the time! Wish I could get rid of 'em somehow...'"This piece of dialogue implies that it's possible to get rid of the Boos roaming the halls, but it's unknown what is received as a reward after clearing the corridors from the ghosts.

Phenomena
The effect the room has on players is opposite from Wet-Dry-World's: the music is commonly an acoustic version of the one used for Dire Dire Docks (although sometimes the audio sample from outside the castle is used), inducing a sense of calm. Some players even reported that they preferred to stay inside the room rather than continuing their playthrough, claiming that they were "too scared to go out."

Sometimes at the bottom of the lake, a 1-Up mushroom is found instead of the usual coin, why it happens it's unclear, but it's worth noting that the more deaths the players had while playing (without counting the times the game over screen appeared), the more likely it was to find the 1-Up.

Reportings
Note: the following reportings should be taken with a grain of salt, as there is no way for some of them to be confirmed.

According to busybody42, it is possible to both visit the Internal Garden and find the Wario Apparition.

How they did so is as follows: they first met Wario, and ran away while they were saying their message. At some point, Wario started to chase them through the corridor. When they left it, instead of the start of the boss fight, they were sent to the Garden.

They were also able to get a star from the Toad after killing three Boos. Before giving the star, the Toad said:"'Thank you for dealing with those ghosts, Mario! Y'know, the other day, I found this lying around. You can have it. I don't need it to be happy.'"

It is unknown how yourbody42 was able to get both the Internal Garden and the Wario Apparition, likely the cartridge had a personalization that balanced between being easier and harder.'' ''

Theorizing
It is thought that the Internal Garden is something akin to a gift that the Personalization A.I. leaves to the players who usually go in into harder levels, stay outside the castle for long periods, or a mix of both.

Trivia

 * The Internal Garden is usually a sign that the A.I. is starting to implement more "benign" modifications.
 * The Internal Garden usually signifies that the game has been, or will be, heavily personalized, similarly to how the White-Eyed Chomp indicates that the A.I. is active.
 * The Wario Apparition cannot manifest if the Internal Garden has been encountered first, and viceversa. However in extremely rare occasions both anomalies can be present (as described above)
 * Cartidges with this anomaly are twice as likely to suffer complications from tilting.