After collecting as many of the Huge Materia as you could, you'll need to go to Cosmo Canyon to move the storyline along. Bugenhagen will talk to you for a bit, and then he'll take you back into his little lab. You'll finally get a chancew to see what these Huge Materia are good for. Touching the blue one will give you Bahamut ZERO, arguably the second most powerful summon in the game. This will only work if you've already gotten Bahamut and Neo-Bahamut. The other ones will give you the Master Materia of the corresponding color if you have met all the requirements (that is, you must have a master-level materia of every Magic materia for green, every single Summon spell mastered for red, and Sense, Morph, Throw, Manipulate, Steal, Mime, and Deathblow for yellow).
Now you must go back to the Forgotten City. After you arrive, take the left-hand path and go into that big room with all the crystals. Bugenhagen will give you some clues and urge you to find a Key. The clue tells you to look "Where sunlight cannot reach". In case you hadn't already guessed, you're going to be looking underwater for this. Cruise around with the sub and head north from Junon. It's easiest to stay below the surface, because you'll have to look for a very small opening that will lead you into a slightly larger chamber. Continue straight and you should find a path that appears to be a dead end. However, if you lower the sub a little and go right you'll find a secret passage. The key is right there at the end. Just drive the sub into it to pick it up. When you've got the key, return to the Forgotten City and give it to Bugenhagen to figure out the signifigance of what Aeris did uderneath the city. Leave the city. On your way out, you'll receive a call on the PHS from Cait Sith, if he isn't in your party already. You'll be told to head immediatly to Midgar, because Diamond Weapon's on a warpath and heading straight towards the city.