World 2-4 is supposed to have only one entrance to the center of the donut-shaped world, but the inner sand barrier has a small gap that you can jump through. It's pretty easy to do, too. First, enter W2-4 and head left towards the first brown grass clump growing in front of the short sand wall. Then, aiming for the wall directly behind the grass, try to jump over the grass, and you should end up behind the sand. Now you can walk around in the empty center circle where the oasis would be. If you keep walking straight back (away from the screen), the camera will move erratically as it tries to focus on you.

While trying to get out of the center circle, it's possible to fall out of the world and into a Game Over if you're not careful. The safest way out is to take the pathway to the oasis. There are also several spots where Mario can get back out as he goes under the wall; just keep walking against the wall until you see Mario's hat poking through the sand platform, then jump to get on top. When you're under the sand platform, you'll fall out of the world if you walk too far away from the wall.