Inside the Fountain

In certain 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. Although this must be done by ground pound on the edge of the fountain. Once in 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 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.

Gusty, Gusty Greenhouse
The level appears to be a giant Greenhouse. The level itself has a wind mechanic similar to tall tall mountain. 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, possibly for a star.