CONTAINS SPOILERS

Jefferson in The Stranded Sailor gives you a clue: "To the West of the Cemetery, behind a rock you will find a passage..." To find it, enter the Lost Cemetery door. Take the first ladder southwest and go all the way up. Cross the long bridge west. Go northwest until you reach the Stranded Sailor sign. Go down the ladders by the cave entrance to reach the passage. Use the Bomb Spell.