In Delfino Plaza, get the turbo nozzle and go to the fruit stands. Stand in between the chili pepper stand and the pineapple and papaya stand. Now, use the turbo towards the wall behind the pineapple stand (go towards the window that's under the awning of the stand). You should run along the wall and go through it at the spot where the door is. You'll land beneath Delfino Plaza. Jump repeatedly or you'll spontaneously die. Also, you may accidentally jump into an underground pipe.
Submitted by: ifonlyicouldremember