Once the player obtains the Morph Materia, which when used, will attack an enemy, and if the enemy dies to the Morph attack, becomes an item, a world of opportunities opens up. Most enemies can be morphed into simple things such as Potions or Phoenix Downs, but a select few enemies can actually be morphed into more Sources, for an almost infinite supply that will allow for the maxing out of stats for every party member, if one so wishes.
Some of these morph-able monsters can be found around the Gongaga area, however the majority of them are found in The Sunken Gelnika. Just about every monster found in the sunken aircraft can be morphed into a Source item and thus used to make the party stronger. This is extremely useful, because the max level for any character is 99, and by the time one is maxed out, his or her stats will only be somewhere in the low 100s, and Source items are required to boost them any further.
Within The Sunken Gelnika, the monster known as Unknown 1 can be morphed into a Power Source in order to raise Strength. The monster classified as Unknown 2 can have the Morph Command Materia used on it in order to obtain a Guard Source, which raises Vitality or Defense. Unknown 3 can be morphed into a Magic Source for stronger Magic Materia damage, while Poodlers, also located within the Gelnika, can be morphed into a Speed Source in order to hasten the ATB. Finally, the Serpent within the Gelnika can be morphed into a Mind Source in order to decrease damage from enemy magic.
Overall, each Source is individually important, and combined together can create an extremely strong character or team, much stronger than simply reaching the maximum level. In fact, it is almost vital to increase these stats via using the Morph Materia on Sunken Gelnika monsters if one wishes to challenge the likes of Ruby or Emerald Weapon.