The Starry-Night Skies

The Starry-Night Skies is a supposedly unused level found in some personalized copies of Super Mario 64. It is associated with the broken painting on the Second Floor of Princess Peach's Castle that uses the Third Floor texture, supposedly being tied to this unused level from when the game was much bigger. Some players have reported being able to jump into it, similarly to the Sun and Sky Seas. This level reportedly only appears during night if the Internal Clock is active. The Starry-Night Skies itself is described to be a large level made up of blue clouds in a night-time sky that requires the Wing Cap to navigate with a theme made from a music box that is reminiscent of the Yoshi's Island opening.

Geography
The Starry-Night Skies is described as a vast cloudy landscape taking place in a night sky, appearing very big and ambitious, potentially due to Major Level Expansion. This level reportedly uses several unused assets and is very hard to navigate, requiring the abilities of the Wing Cap or some other anomalous power-up that allows for flight or access to far away platforms such as the Tanooki Cap or Safety Cap to reach other cloud platforms, giving it a very unique feel.

There are various unique structures around the level, such as towers, a flying ship that is commonly destroyed and appears much more like a ghost ship, a larger cloud and a wishing well, typically containing something notable, such as a mission. These areas are also scattered with enemies, such as Fly Guys, Goombas, Lakitus, and areas specific to certain areas, such as Boos and Scuttlebugs on the flying ship and even King Whomp on a large cloud in some reports, either dropping a Power Star or just a red coin, similarly to his reappearance in Tick Tock Clock. Most enemies reportedly have Enemy Discoloring to fit this level's night time theme, adopting a dark bluish tint most of the time, along with star patterns.

Missions
Several stars have been reported in this level, such as a star simply being found in the sky while flying around, collecting 8 red coins, defeating King Whomp, defeating a Big Boo on the flying ship, sometimes only spawning upon defeating every Boo like the first mission of Big Boo's Haunt, climbing a tower, collecting 5 secrets throughout the level, collecting several coins and putting them in the wishing well, racing a Klepto-like character that resembles a stork or simply running into them to get a Power Star, and a 100 coin star. Notably, many reports claim references to an "Eternal Night Star" that is consistently never found. Due to this level's anomalous nature and the use of the word Eternal, this may be the supposed Eternal Star.

Theorizing
Because of this level's theme, the music box music, the stork, and painting, it is believed this level was a reference to Yoshi's Island, which was released a year before Super Mario 64. This may mean Yoshi was also intended to appear in this level, or that other games such as Super Mario World would have been referenced much more as well, potentially explaining several other anomalies from previous games.

If this level is legitimately unused and not created by the AI, then it might be the 32NIGHT level, as judging by the name, it would have been night themed, and this level takes place in a night sky. However, if this was the case, why 32NIGHT does not show the Starry-Night Skies when loaded is unknown, likely due to the assets being replaced after it was scrapped.

Sunny Skyline
Sunny Skyline is another anomalous level associated with the Starry-Night Skies, as well as the Internal Clock and Internal Weather System. If the player is playing during the day and the sky is clear, some have reported that Starry-Night Skies' painting has been replaced with Sunny Skyline, a day-time variant of the Starry-Night Skies. It reportedly has a much more positive emotional aura, without Enemy Discoloring, and the flying ship is commonly a regular ship instead of the ghost ship, removing the Big Boo mission and instead making it much more similar to Rainbow Ride's airship mission. However, the level is also much smaller, potentially being unrelated, and created by the A.I. instead of being created by the game's developers. This level has been compared to Moonlight Isles in that they're both inverses of the other level's time of day.

Lunar Landscape
Lunar Landscape has reportedly appeared over the Starry-Night Skies painting, typically if the Starry-Night Skies and/or Sunny Skyline do not appear or if it is detected that is a full moon. However, it has also appeared on other paintings on the second floor instead of replacing the Starry-Night Skies, so it is probably its own level.

The Trial
The Trial is a supposed anomalous level that is potentially a Challenge Course that unlocks the Starry-Night Skies, only appearing to people who are "worthy", similarly to Hollow Horror Halls. The Trial can reportedly appear behind the waterfall in the Castle Grounds, or some other hidden area. This level reportedly takes place in an expansive cavern similar to Hazy Maze Cave that the player has to navigate, or a much more maze-like version of Princess Peach's Castle. A strange commonality in reports of the Trial is a reported lake with Dorrie in it nearly identical to the one seen in HMC.

Marionova64's Documentation
Marionova64, most popularly known for documenting Bowser's Domain, has also shown an area similar to the Starry-Night Skies by entering the same painting, though this is likely a separate level due to its stark contrast with the common description. The video begins with Mario standing in front of the painting, which has been changed to be an actual painting, before jumping into it and entering a bizarre version of Tower of the Wing Cap with darker textures, storm clouds, Bowser in the Sky's skybox, no red coins and no Cap Switch. This has been reported to be a night-time version of TotWC created by the AI, though due to appearing in the same painting as the Starry-Night Skies has been grouped with it. The player flies around, before flying into the void and being warped out as the game ends.

SM64DD Developer Anecdote
In one of the many posts from the developer of the scrapped Super Mario 64 Disk Version, the developer referenced what appears to be a new version of the Starry-Night Skies planned for SM64DD called Afar Star Skies. This level was even reportedly made from unused assets, supporting the theory that this is the newer version of the level. Afar Star Skies itself is supposedly made up of large mountains stretching up into the sky with cloud platforms reminiscent of those from Wing Mario Over the Rainbow. The player would start at the bottom mountain and platform up to reach the central mountain. There's also a Toad House on one mountain, a cannon on another, and a Wing Cap box on yet another mountain. In the first mission, there was supposed to be a boss, though King Whomp was used as a placeholder, potentially due to their appearance in the original Starry-Night Skies. Enemies had also not been added, though a unique obstacle was described, being raining shooting stars similar to the Water Balloons from Bob-omb Battlefield. 3 other missions were also described: Talking to the Toad in their house, finding 5 secrets in bushes, and using the Wing Cap to fly to the cloud at the edge of the map to find the Power Star.

Trivia

 * While it could be a coincidence, the placements of the stars on the painting seem to line up with the notes for the "Song of Storms" from the Legend of Zelda: Ocarina of Time. Ocarina of Time has paintings of Super Mario characters hidden in an inaccessible room, and the games were in development at the same time, so the resemblance being an intentional reference is a possibility.