Get up to the highest point on the wires in the hills. It should be near the edge of the level (away from where you enter it) with a trail of coins above it. Once you're there, go to the far end of the wire that faces the border of the world (Mario's back should be facing the beach). Align the camera roughly behind Mario. Jump off and hover forward until you get frozen against the invisible wall. You may hold up on the control stick to stay, but fall to go through the hill and below the level. You may end up in a stuck underwater area, or you may be able to walk around. If you stand around too long, you'll die, so jump a lot.
Submitted by: Diged