In Pianta Village (the level with the catwalks and mushrooms underneath the "main village") select Episode 5 (secret of the Village Underside) get Yoshi and then head down towards the mushrooms as normal, now if you fall off of one of the mushrooms , with Yoshi's hovering skills you can "slide" down the root of the mushroom all the way down the root. If done properly, you can now run around on tan invisible floor way way below the level. The best part is (other than being somewhere the programmers didn't expect you to be able to go is that you can PAUSE and hit "EXIT AREA", which saves you a life you otherwise would have lost. And it looks damn cool too.