This one is a little tricky. In Super Mario 64, go to the courtyard, and get a bunch of boos to collect in the fountain. Now, on the back of the fountain, do a backwards long jump, and land on a boo. You will bounce, and still be in a long jump. Keep bouncing until you land on the tiny ledge next to the star, on the back. Quickly do the Backwards Long Jump Trick. You will shoot through, and fly behind where the door is. (you have to keep doing the jumps) Now, you are in a Dark World very similar to the original. There might be a way out, but the video didn't show any.
Why it works Edit
The backwards long jump shoots you past the wall. Other than that, It's like Dark World 1.
Found in a glitch video.