In Search of Holy
Head over to Cosmo Canyon and talk to Bugenhagen. He'll explain a couple of things, then you'll give him the Huge Materia for safekeeping. You can always return here for them, and for more info on them, see the section on Other Materia.
Next, Bugenhagen joins you aboard the Highwind to help you search for Holy. Head back to the Submarine and find an underground tunnel southwest of the North Continent. At the end of that tunnel, you'll find the Key to the Ancients.
Next, hop aboard the Highwind and fly to
the Forgotten Capital. Land your plane right next to it (the only
place you'll be able to land that's not grass) and head to the
crystal you saw before. Bugenhagen fiddles with the key and fits
it into a music box, causing a spectacular waterfall to appear.
Head to the crystal, which is actually a sort of projector, and
you'll view Aeris praying. After some conversation, Cait Sith
informs you that Sister Ray Cannon has been moved to Midgar. Exit
the Forgotten Capital and enter the Highwind.
Head over to the Midgar area and you'll
find the Diamond Weapon heading straight for the city. You'll
have to wait until it completely emerges from the water to
challenge it to battle. Use lightning-based spells on it and
Ultima. Then, after you've defeated it, it sends a barrage of
missile-like energy projectiles straight for Midgar, killing
Rufus in the process. As a counterattack, though, Sister Ray
sends a great beam of energy at it, killing it and destroying the
barrier around the North Continent as well.
Next Section: Final Assault on Midgar
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Section: Space Travel