To obtain the Huge Materia in Rocket Town, press Circle, Square, X, X as the passcode.

There are four huge Materia that can reproduce other materias if you can pass the requirements.

Blue - You must have Bahamut and Bahamut Neo. Take these to Cosmo Canyon and talk to the Blue Huge Materia and you will recieve Bahamut Eishiki (Zero).

Yellow - Master the Steal, Throw, Mime, Manipulate, Morph, Deathblow and Sense Materias, then talk to the yellow huge Materia to recieve the Master Command materia.

Red - Must have all 16 summoning materia at master level then talk to the red huge materia to recieve the Master Summoning materia.

Green - Have all 21 magic materia at master level then talk to the green huge huge materia and you will recieve the Master Magic materia.