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There are exact TWO ways to obtain the Materia.
1. Get the Bahamut Materia in the Temple of the Ancients and get Neo Bahamut in the Whirlwind Maze. BOTH can be missed if not picked up. After both are in your pockets go and save ALL of the 4 huge materias in the side quests for them.
The first Huge Materia is found in North Corel as soon as Cid becomes the party leader. The Huge Materia must be taken from a runaway coal train.
The second Huge Materia is won after the battle minigame in Fort Condor. You dont need to win, simply face the boss. If you lose to the boss Fort Condor is defeated and the Huge Materia is lost.
After these two missions, travel to Mideel, Cloud becomes party leader again and can start the next mission.
The third Huge Materia is found on the Bottom of the Sea near Junon on the sunken red submarine. If you fail to destroy the red submarine in the torpedo minigame, the Huge Materia is lost. But trust me its nearly impossible to lose.
The final Huge Materia must be saved from the Shinra No. 26 rocket while in space by inputting a code. If you fail to guess the code in time, the Huge Materia will be lost.
The code is everytime:
,
,
,
If you never guess wrong then Cid will comment it with a funny quote.
After obtaining all huge materias and both Bahamut materias inspect the blue huge materia in Cosmo Canin in Bugenhagens room. You here a growl and the Bahamut-0.
2. If you fail to get all 4 huge materias or fail to geht both Bahamut Summons, Bahamut-0 can be digged out in Bone Village late in the game. The chance of digging this out is around 4%.
here is a map
it lays by the Orange spot on the map.
Full credit for the picture goes to cetraconnection
Edit: Changes