Instructions
Fly to Pacifidlog Town.
Surf west on Route 132. While surfing, stay as far south as possible so you stay out of the heavy current and avoid hitting too many land masses.
Surf west again on Route 133 and 134. You will see a large area of shallow water between four rocks. In the center there is one square of deep water.
Enter the deep water square in the center of the shallows.
Dive down into the water. Follow the underwater cavern (there is only one path) and dive again when you reach the center of the back side of the path.
Walk to the back of the sealed chamber where it looks like there has been a cave-in. Dig there to open up the cave.