Note that this is the second visit to Rocket Town on Disc 2. If you are on Disc 1 and if this is your first visit to Rocket Town you should proceed to that page of the walkthrough.
Before proceeding with any of the side quests or with the story for this part of the game you should make sure that you have already picked up the Key to the Ancients from the underwater cave and picked up the Huge Materia from the destroyed red Leader Sub after the submarine battle.
Return to the Underwater Submarine Battle page and check near the bottom for more information.
There are three additional side quests that you should take care of before returning to Rocket Town:
Lucrecia’s Crystal Cave
Locating Vincent’s lost love, Lucrecia and obtaining Vincent’s Ultimate Weapon and Level 4 Limit Break.
The Sunken Gelnika
The sunken transport plane found in the bottom of the ocean. It contains some of the games strongest enemies as well as a huge assortment of end-game items.
The Da-Chao Fire Cavern
Returning to the Da-Chao mountain range with the Leviathan Scales to pick up a weapon for Yuffie and Steal as Well Materia.
Return to Rocket Town after you have completed all of the side quests listed above. Travel to the Weapon Store and pick up the 
Fourth Bracelet and to the item shop to pick up a 
Guard Source if you have not done so already. Proceed to the back towards the Shinra No. 26 Rocket.
You will be thrown into battle against the Shinra Soldiers once you get close enough to the stairs in front of the Rocket. There will be two more battles against Shinra Guards and Soldiers that occur automatically in succession after the first battle but they are all relatively easy fights.
Cid will automatically join your party after the last battle. There is only one more boss fight coming up against Rude and it is fairly easy, so don’t waste too much time equipping proper Materia to Cid. Climb up the ladder to initiate the last battle.
HP: 9,000
Reward: Hi-Potion
This fight can barely be qualified as a boss battle. Make sure that you have 
Steal Materia equipped on someone and that you steal another Ziedrich armor from Rude before you end the fight.
Enter the Rocket for one last fight against a “Senior Grunt”. Continue into the cockpit to continue the story.
The team will then be locked into the Shinra No. 26 Rocket and shot into space. There are a number of quick cutscenes involving Cid and his crew members.
The doorway to grab the Huge Materia is on the right side of the screen (as Cid will kindly point out to you if you try to go the wrong way). Climb the ladder through the duct to get to the Materia Room.
Your first attempt at entering a password on the control panel will start a timer sequence. You have until time is up to get the password correct or this piece of Huge Materia will be lost forever (and cannot be retrieved later in the game).
The correct password is: 
Proceed back down the ladder in the duct towards the cockpit and then back towards the ship hallway. There is a ladder close the camera in this hallway that leads to another duct. Proceed through the Engine Room and another fairly lengthy cutscene will take place. Follow Shera towards the Escape Pod.
Bahamutype-0
In order to obtain the 
Bahamut ZERO Materia later on in the game, which will reward you with the Bahamutype-0 Trophy as well, you need to obtain a number of items including each of the four Huge Materia with this one being the last one of the four.
You may want to consider restarting your game if you were unsuccessful in cracking the code and releasing the Huge Materia in time as there is no way to obtain this trophy unless you retrieve it.
The Shinra No. 26 Rocket crashes into Meteor but unfortunately only ends up splitting it in half.
Red XIII will suggest returning to Cosmo Canyon to meet up with Bugenhagen. Before you leave this area though you should revisit Rocket Town one last time. Refer to the revisiting Rocket Town section for more information.
Jump back into the Highwind and fly over to Cosmo Canyon. Enter the town and walk back up the stairs towards the Observatory. Enter Bugenhagen’s contraption from the entrance on the main floor and speak to him to continue the story.
Bugenhagen will offer to keep the Huge Materia in a safe place, i.e. the machine at the top of the observatory. You will now be rewarded if you managed to save all four pieces of Huge Materia.
Depending on how many missions you completed you will see the following Huge Materia:
Examine the Blue Materia and choose to “Get closer…” when prompted and you will receive the Bahamut ZERO Materia.
This Materia can only be obtained if you obtained all four pieces of Huge Materia. You can return to the underwater world in the Shinra Sub to pick up that piece of Materia if you missed it but if you missed the Huge Materia in North Corel, or if you did not correctly guess the code on the Shinra No. 26 Rocket or win the battle at Fort Condor you will not be able to obtain the Bahamut ZERO Materia.
Bahamutype-0
If you picked up both Bahamut Materia and retrieved all four Huge Materia you will now be rewarded with theBahamut ZERO Materia. This will also reward you with the Bahamutype-0 Trophy.
Choose to leave the Observatory after you look at the Huge Blue Materia and you will automatically appear on the Highwind with Bugenhagen on board. The next destination is the Forgotten City.
