Get Luigi, and then enter the second star mission of Tall, Tall Mountain. Go up the mountain by the way of Ukiki, the shy guy, and the rolling log (taking the owl doesn't usually work, though sometimes it does). Once you are at the top, past the waterfall, there is a red box. Get the flower and become invisible. After the invisibility wears off, or even as it is wearing off, work your way down the mountain to the area where there are several large red polka-dotted mushrooms with red coins on them. The second-smallest of them is a teleporter; when you stand on it, it will teleport you to an area along the cliff near the cannon. If done correctly, when you get teleported there, you should be underwater, and can swim. When you swim off the edge, the wind will blow you and ruin the swimming physics but keep the blue tinge to the level. Swim almost straight up and keep close to the mountain. Once you are past the ledge, you can swim around and there is a top to the water where you can get air and then dive back under and keep exploring. If you go near the start of the level, the underwater glitch turns off.
Submitted by: ..DS..::Dude