This small, triangular island holds one of three goddess statues. You must place one of the three colored pearls on the statue to raise the Tower of the Gods from the Great Sea.
Octo Chart (via Treasure Chart #26)
Find and open Treasure Chart #26 (found at the Six-Eye Reef [D4]) to pull a chest from the Great Sea near Northern Triangle Island. The chest contains the Octo Chart.
Rings of Light
Four rings of light around this island appear only under the light of a full moon. Each has a chest that contains a Red Rupee. When you see these rings, use the Grappling Hook to pull up the chest and grab the Rupee.