Super Mario World/Donut Plains 2

This is both the game's first underground level, and the first constantly-scrolling area (aside from the last portion of Iggy's Castle). There's lots of Buzzy Beetles and Swoopers. The secret exit that leads to the Green Switch Palace can be reached by entering the green pipe that's after the green block among the rising yellow stones. The four yellow blocks that form a staircase lead to a chamber that holds the key. The last block has a vine in it, directly above is the entrance to the chamber. Flying in just with the cape can be tricky since the entrance is narrow, so it's better to let the vine grow and grab it. There's a couple different ways to get the vine to grow: you can fly up to the block and hit it, jump off of Yoshi in midair, or simply toss up the blue koopa shell that's nearby.