Warp Zone (Super Mario Bros. 3)
From the Super Mario Wiki
The Warp Zone, also known as World 9, is a secret world in Super Mario Bros. 3. Unlike other numbered worlds in the Mushroom World, it is not accessed by clearing the previous World 8. No actual levels exist in this world, and it only serves to skip to another world. The game cannot be saved while in the Warp Zone.
The only way to get to the Warp Zone is to use a Magic Whistle. Where the player ends up is dependent on the world they used the Magic Whistle in. If the Magic Whistle is used in World 1, the player can choose from Worlds 2-4. If the Magic Whistle is used in Worlds 2-6, the player can choose from Worlds 5-7. If the Magic Whistle is used in Worlds 7-9, the player can only choose World 8 (thus using another Magic Whistle while already in the Warp Zone will lead to World 8 section).
The Warp Zone consists of a small island, littered with Warp Pipes leading to other Worlds. Any other geography cannot be defined, due to there being no actual levels. No inhabitants or rulers live on this island. The island that is close to Desert Land, Water Land, and Giant Land on the GBA version's World Select screen may be the Warp Zone, due to it being a very small island with the same color scheme as the Warp Zone.