Mushroom Kingdom II
From the Super Mario Wiki
Mushroom Kingdom II is the second Mushroom Kingdom stage in Super Smash Bros. Melee. It can be unlocked by collecting the Birdo or the Pidgit Trophy. Despite the name, this stage is actually in Subcon, from Super Mario Bros. 2. The stage has a platform in the middle, and two on the side. A waterfall forms huge gorges that separate the three. Occasionally, logs will fall down the waterfall, creating temporary platforms. A Pidgit will occasionally fly in, making another platform, and a Birdo also appears occasionally, spitting Birdo Eggs at the characters.