Things You'll Need
Instructions
Play through the game until the party takes control of the airship called the Highwind and the creature named "Ultimate Weapon" attacks the city of Mideel.
Go the town called Junon and enter the underwater reactor through the elevator.
Fight in random battles in the reactor until the party is attacked by a monster called a "Ghost Ship."
Use the "Morph" materia on the Ghost Ship to transform the creature into an item called the "Guide Book."
Leave the underwater reactor and travel to the city called Kalm. Go inside the house at the eastern edge of the city and speak with the man inside. He will want to trade specific items with your party.
Trade the Guide Book for the "Underwater" materia.