Once you reach the final stage of the game, Genesis' secret hideout in Banora, you must get the Seven Goddess Materia and set it on the pedestal in the Portal of Severance. However, there is an optional section. You can reach it by defeating all the monsters in the Lake of Oblivion. To do this, check all the Lifestream Springs. When Zack says "The Lifestream has ascended and crystallized", it indicates that you have to fight another enemy first. One of them will make an enemy appear. After defeating it, check the remaining ones again to find the one that spawns the next enemy. Repeat this until all the springs disappear, and a pathway will appear in the back. It will lead to an optional section. Here you must defeat enemies to gain keys to the doors so that you may advance. The purpose of this section is to obtain some extra items in the cells. The cells can be opened by turning the valves on the walls next to each cell. Some of them do not have valves. If it has a hole in the wall, you must find a hole that leads you into that cell. If it does not, it is inaccessible. Once you finish this section, you should have gotten the "Iron Bars Key". Use this on the door with the Behemoth behind it. Fight the Behemoth, then use the door behind it to appear in the Portal of Severance on the second floor. All you can do is take the elevator down to the first floor.