Dune Bud Sand Castle

From the Super Mario Wiki

The Dune Bud Sand Castle is a secret area in the game Super Mario Sunshine. This area appears in Gelato Beach, and can only be accessed by Mario spraying a certain Dune Bud with FLUDD. Many Dune Buds are sprayable and will cause enormous sand structures to emerge from the ground, but when this particular bud is sprayed, the sand castle emerges far away. The castle mostly consists of a castle-shaped arch with nothing behind it, but when Mario walks through the arch, he enters the secret area.

This particular secret area is mostly made up of numerous sand blocks in which Mario must hop to and fro. The sand blocks disintegrate went Mario steps on them, so he must use speed and agility throughout the stage. Several 1-Up Mushroom]s can be located around the area, and at the end, on top of a large sand block tower, is a Shine Sprite.