Koholint Island is full of hidden passages. Look for cracks in walls that can be bombed, suspicious-looking bushes that can be cut or burned, and discolored patches of ground that can be dug. Many Secret Seashells and Pieces of Heart are hidden this way. Pay attention to hints from NPCs, as they often subtly point towards secret locations. The Magnifying Lens, obtained from the trading sequence, reveals hidden text that can lead to a secret dungeon.
