In the overworld as Chunky, become Hulky Chunky and run over to the main island. Go to the large chunks of land that lead to the tree that lead to Angry Aztec (the tree won't be there, though, since you're Hulky Chunky). Jump onto the first "step" and then try jumping into the second. Eventually, you'll fall through. Become small. Now, you can do a lot. For example, you can walk underwater for a bit. If you go too far, you will start swimming. However, if you swim to an area that does not seem to have water on the surface, swim to the surface, and then jump, you will fall glitchingly and land on the ground, able to walk some more. Going too far out will get you out from under the world. If you swim up to the ground under the mainland, you can attract the attention of a Gnawty and if it runs at you, it will hit the small bit of your head popping out and hurt you - making you fall and walk underwater some more.