Here's how you can go to the minus world. You must be at least big for this to work. Towards the end of level 1-2, there is a pipe sticking out from the wall. Stand on top of the pipe and break the second block from the right. This next part may take a bit of practice. Facing left, jump up where you broke the block and try to move right, but not breaking that one brick by the wall. You want to hit the corner and sort of go through it. If done right, Mario will walk backwards for a little bit through the wall, and over to where there is normally a warp zone. The pipes will have piranha plants in them and the pipes won't say which pipe is which. The pipe in the middle is 5-1, and the pipe to the far left is -1, and I think the pipe to the right is also -1. Once you take the pipe to the left, you'll be in -1. There's nothing good there. It's a water board, with a pipe at the end. The pipe takes you glitchishly back to the beginning of -1, so it never ends. There are also other negative levels, zero levels, and letter levels which you can get to through game genie codes.
Submitted by: Brett Notter
A Youtube video of this glitch can be seen here.