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The code to get that huge materia is circle, square, x, x. When you get that huge materia take it to Bugehagen's observatory and provided you have the other huge materia already there you will be able to get Bahamut Zero by examining the huge materia. There are 4 huge materia in total and the locations for them are:
Corel- you will get this one during the story.
Underwater Reactor in Junon- you go here during the story but you will need to go underwater in the submarine afterwards and locate the Shinra submarine you destroyed to get it.
Fort Condor- this one is optional all you need to do is defeated the Shinra army attacking the reactor and then go out to the reactor where you will get the Phoenix materia. When you go down stairs and talk to the leader in Fort Condor you will find out that he removed the huge materia from the reactor and that he has it himself, he will give it to you after you are done talking.
Rocket Town- just enter the password(circle, square, x, x).