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. Strangely, there is no Bowser level inside the fountain. Here are some of the levels that appear here.

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 two levels are known so far. Please note that further investigations of this area will be conducted in order 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 behaved 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 somewhat similar to the levels involving Plessie in Super Mario 3D world.