In Noki Bay, once inside the bottle in the Red Coins in a Bottle level, Mario should fall to the bottom of the bottle and then locate a small rock formation there. If Mario maneuvers into a small square opening towards the back right of the rock formation, he can find a small door inside the opening which is impossible to open. Therefore, the player must rotate the camera to see what is on the other side of the door. In doing so, Mario can see a brown book on the floor of the room behind the impassable door, though its purpose is unknown.

The book itself may be a reference (or vice versa) to the level's background music. In the Original soundtrack, the title of the level's theme is in fact "The Book in the Bottle". It's also been said that this mission was originally different, and that Mario would collect the book rather than 8 red coins.
Contributed by Bean101
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