Inside the Fountain

In some personalized copies, the courtyard can contain a secret area similar to the 4th floor and floor 2B. This area being seemingly inside or rather, underneath the fountain. To access this, Mario must ground pound inside the fountain, which will break it open and will take him to this area. Once Mario is inside the fountain, more levels will open up. They can be accessed in any order the player wishes, just like with the 2nd floor.

Geography
When Mario drops into the area he's inside a small room with 4 sets of stairs, 1 on each wall. Going down any of these stairs appears to take Mario to the same square shaped hallway, but on a different side. The hallway itself also appears to have aquariums with fish in them like the room for Jolly Roger Bay. On one side of the hallway is a door that leads to a room with paintings of levels in it. In the hall opposite to it is the painting to Gusty, Gusty Greenhouse.

Levels
Currently only four levels are known so far. One of them being under investigation right now. Please note that further investigations of this hub area will be conducted to get more information about it.

Gusty, Gusty Greenhouse
The level appears to be a giant greenhouse. The level itself has a wind mechanic similar to Tall Tall Mountain, although in a much larger quantity. However, there is not much to note about except for a few things. There appears to be an unused sunflower NPC that looks similar to Amazy Dayzee from Super Paper Mario. For an unknown reason, MIPS appears in this level. Later games would involve catching rabbits for stars, so MIPS may have intended to behave in a similar manner.

Steam Bubble Cascade
The warp to this level is inside the gallery, with an entrance similar to that of Hazy Maze Cave's. Another aspect of Hazy Maze Cave apart of this level is the Cave Dungeon song. Steam Bubble Cascade is a rather large level with a few things to note. Firstly, part of the level seems to be in reference to Donkey Kong. There is a large pathway with barrels that Mario must jump over, possibly with a star at the top as a reward. Next, there seems to be an unused effect for steam, with the purpose being to obscure the player's vision, causing enemies to get a surprise attack on the player, without the player being able to see where they came from. A star in this level involves going behind the waterfall in the level and solving a puzzle that involves ground pounding stone slabs into the ground in a certain order.

Finally, the sixth and final star in the level is called "Joyride in the tunnel" In it, Mario must find an entrance hidden in the lake, once entered Dorrie will greet him and say something among the lines of this:

"Oh, hey Mario! Heard you were looking for stars. I think I saw one at the end of this tunnel.

I can take you there but be careful! Bowser's minions are flooding this place!"

Once Mario gets on Dorrie's back, Enemies such as Goombas, Bob ombs and even Lakitu (Enemy) will attack Mario. Something notable here is that this appears to be the only other level that Spiny appears in other than Tiny-Huge Island, Strangely, this is the only mission Spiny appears in. Once Mario survives the onslaught of enemies for about 1 minute and 30 seconds, Dorrie will arrive at the star and Mario is now able to get it. This mission appears to be similar to the levels involving Plessie in Super Mario 3D world.

Chilly Cactus Mountain
Chilly Cactus Mountain is another level which is also in the gallery, The painting to the level is in it's own room in the gallery. Interestingly, the level has two entrances that lead to a different part of the level, not to different from Tiny-Huge Island. Before entering the level, Mario can choose to go down one of two hallways, an orange hall, or a blue hall. The Orange hall's painting takes you to a desert section of the level while the blue hall's painting takes you to a tundra section of the level. Into the level itself, Chilly Cactus Mountain rather unique level that has two sections in it. A desert section and a tundra section, as mentioned before. Two stars appear to be in each section, it's currently unknown how the sixth star is achieved. However, the first star involves Mario climbing up to a tower of sorts that rests in the middle of the level. Interestingly, the unused Cactus sprite appears in the desert part of this level. Along with unused small chill bullies in the tundra part of the level. Strangely, using the metal cap in this level has Mario appear gold instead of metal. It's unknown how this is caused.

Molten Magma Cavern
This level is currently under investigation, more info will be released soon.

Bowser in the ??? ???
Despite the first investigation, a Bowser level was recently found in the fountain. However as of right now, it is impossible to access. The star door rests upon a platform in the gallery that's not only above the door leading to the gallery, but also the perfect height for even a triple jump to just miss. What's more, the star door is also behind what appears to be a cell with a warp pipe inside of it. As of right now, this level cannot be accessed. However, one thing is certain, Mario must find a warp pipe either in the gallery or in a level. Once this pipe is found, the Bowser level can be accessed.