From the Super Mario Wiki
Mushroomy Kingdom is a stage that appears in Super Smash Bros. Brawl and returns in Super Smash Bros. for Nintendo 3DS. It is based on World 1-1 of Super Mario Bros., but with a more barren and realistic look, rather than the typical 8-bit graphics used in previous Super Smash Bros. installments. The stage is an almost exact replica of World 1-1 from Super Mario Bros., with the stage gently scrolling by. Due to this, players have to keep moving with the stage, as well as KO their opponents. There are many blocks, pits and walls to act as obstacles along the way. Other than this, there is no real danger, as the stage scrolls by slowly. When the stage is selected, it will sometimes load up as World 1-2 from Super Mario Bros., an underground level. The same conditions apply, only the stage appears differently.
Several small changes were made to facilitate Smash Bros. gameplay: the Goombas, Koopa Troopas, Piranha Plants, Coins, and World 1-2's Warp Zone were removed; the Warp Pipes that led to bonus rooms and the flagpole of World 1-2 were also made inaccessible. Blocks that held Coins and power-ups were changed to release items in the stage, and Coin Blocks now offer up to three items. In World 1-1, the hidden 1-Up Mushroom Block was lowered by a block, the flagpole and fortress are in the background, and the block the flagpole stood on was removed. In World 1-2, the Brick Blocks on the ground after the pipes and at the end of the level were changed to ground blocks in the stage.
Here is a chart for which buttons on which controllers to hold down to ensure which level will load up when selecting Mushroomy Kingdom:
In Super Smash Bros. for Nintendo 3DS, despite the World 1-2 version's absence, the underground song can still be played. The Final Destination form of Mushroomy Kingdom is set on the ground with several blocks on top of it, creating a perfectly even platform. The fortress can be seen in the background.
Super Smash Bros. Brawl
Super Smash Bros. for Nintendo 3DS
Names in other languages