World 4 (New Super Mario Bros.)
From the Super Mario Wiki
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World 4 is an alternate world found in the game New Super Mario Bros. This world is mainly jungle-based, and features a type of purple colored toxic water, which kills Mario and Luigi with a single touch - regardless of what form they take.
This world is accessible by defeating Mummipokey from World 2 in Mini Mario form. World 4 introduces a lot of new enemies, such as Wigglers and Scuttlebugs. It also introduces Dorrie - a friendly creature who helps the Mario bros. travel along the toxic water. As with all the other worlds, Bowser Jr. is fought here. The boss of this world is Mega Goomba, and defeating him will lead the player to World 5.
If the player looks closely on the world map, when Mario walks on the secret path from 4-1 to the 1-Up house, he appears to walk backwards on one of the sections.