The Ancient Machine
Return to Bugenhagen's laboratory in Cosmo Canyon to discuss how to overcome Meteor and Sephiroth. However, this proves uneventful until Bugenhagen suggests that you return to the Ancient City. Before you can do this, though, you'll need to store the Huge Materia in Bugenhagen's lab. This may seem odd, but it doesn have a purpose. Before you leave, take a close look at the blue Huge Materia and you'll receive the "Bahamut ZERO" Materia.
Hop back into the Highwind and make your way to the Ancient City. Once there, take the left path to the back. Bugenhagen explains the use of the White Materia and finds clues to locate a hidden Key. The key is hidden in a place "even sunlight can't reach", which suggests a cave of some sort.
Hop back into Highwind and head for the coast. You've searched nearly every cave in teh outer world, so it's time to explore underwater.
Take the submarine down and head to the south. The Shinra sub you wrecked earlier is liying on the bottom here. Inspect the wreckage, and you'll find the last of Huge Materia.
Warning:
There's a huge creature roaming around this area. It's one of the new WEAPONs that were added to the American version of Final Fantasy VII. Avoid it for now, because it's ultra-powerful and WILL make mincemeat out of your teamuntil your character's levels and Materia are maxed out. If the WEAPON is blocking something you need, surface and then dive again. This moves the WEAPON to another location.
Now head north and hug the west wall until you come to a side passage. In the back, you'll find a wrecked plane that you can inspect, although it's not necessary. (Refer to somewhere in this guide for more detail, I can't remember where I put it, oops... I will insert a link in when I find it).
Go back to the main area and continue north. After passing through a narrow passage, take a hard left and follow the canal to an underwater cave. Surface when you reach the end of the cave and you'll appear in the middle of a lake. Park the submarine along the rock wall and use the PHS to place Vincent into your party (if he isn't already with you). Enter the waterfall to learn more about Vincent past.
Tip:
Visit this spot again during Disk 3 and you'll receive Vincent's ultimate weapon and Limit Break.
After the story, dive again and go back toward the main underwater passage. Instead of going south, continue to the north until you find another underwater cave. Follow it until you see the Key of the Ancients. Take the key back to Bugenhagen at the Ancient City.
When Bugenhagen uses the Key in the ancient device, you learn that Aeris has already prayed for Holy; however, Sephiroth's presence keeps Holy from moving. If Sephiroth isn't eliminated, Holy will be unable to stop Meteor destroying the planet.
As the team leaves the Ancient City, Cait Sith reports that the Junon cannon has been found. Shinra has attacked it to the Mako Reactors at Midgar and plans to use it to destroy Sephiroth. The team had better return to Midgar- and fast!
WEAPON Attacks
WHen the team reaches the Highwind, the entire area starts to quake. Another WEAPON is on the move and it's headed straight for Midgar. Fly back to Midgar and land the Highwind near the nmorthern coastline. Find a good spot on the beach and wait for it to arrive.
Boss Fight: Diamond WEAPON
Diamond WEAPON is one tough customer, but the team should be more than a match for it. Start the fight by casting MBarrier on the entire team, and then cast Regen and Haste. You may also want to cast Barrier to reduce the damage from Diamond WEAPONS's physical attacks.
Attack Diamond WEAPON with your most powerful spells and summons. Ultima, Comet, and Knight of the Round work really well. Diamond WEAPON can't be harmed by physical attacks, so stick to magic unless you're trying to mug the beast. Beware when Diamond WEAPON begins to count down, because this means it's preparing for its super attack. The Diamond Flash does about 1500 to 3000 points of damage to each character and mutes those who survive, which makes it difficult to quickly heal your team. You may want to equip a team member with the Ribbon accessory to protect him/her from the Mute effect. That way, when the countdown reaches "1", you can hold off that character's action until after Diamond Flash connects. This enables you to quickly cast Cure 3 and save anyone else from getting picked off by one of Diamond WEAPONS's weaker attacks.
After the battle, Diamond WEAPON is stunned but is still moving toward Midgar. Shinra's Cannon, Sister Ray, is up and running and prepared to fire - but so is WEAPON. The blast from the Sister Ray cuts through WEAPON and destroys Sephiroth's barrier, but not until WEAPON has already fired upon Midgar.
Hop in the Highwind and head north. Sure enough, here's nothing stopping the team from entering the crater. However, Cait Sith alerts the team that the Sister Ray is overloading and preparing to fire a second shot at the crater. Hojo has taken control of the cannon and there's nothing that Shinra can do to stop him.
Head to Midgar but fly over it instead of landing. The team dons parachutes and enters Midgar commando style.