WORLD -1
Sharp readers already spotted that an example of how to get to the "minus world" was shown in the "Application: Jump into a wall just below a solid ceiling and walk through it" chapter of this page.
The minus world is basically just a random location of the game ROM, it is not a real world. It wasn't designed that way ― it's just an oversight of the programmers that you can get into the pipe before the warp labels are assigned.