From the Super Mario Wiki, the Mario encyclopedia
The SMB2 Mushroom (short for Super Mario Bros. 2 Mushroom) is a power-up introduced in the version 3.0.0 update in Super Mario Maker 2 released on April 22, 2020. It is exclusive to the Super Mario Bros. game style. In the nighttime ground theme, SMB2 Mushrooms bounce whenever the player jumps.
Similar to the Superball Flower and Master Sword items, the SMB2 Mushroom also causes music from Super Mario Bros. 2 to play, and changes music depending on the course theme. When using the SMB2 Mushroom in the ground, sky, underwater, forest, desert, airship, and snow themes, the music is replaced by the overworld theme from that game; and in the underground, ghost house, and castle themes, the game's underground theme plays. Sound effects are also borrowed from the game. The boss theme sound effect while in SMB2 Mario is the boss theme from that game, while the final boss sound effect is Wart's boss theme. The bonus game theme uses the source game's character select music, while the Peaceful theme uses the music of the source game's ending. P Switches use the source game's Subspace music, while the sound effect of a thrown POW Block is borrowed from the same action in Super Mario Bros. 2 as well.
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