My money's still on Cloud opening the tank.
We know you don't chase the real Sephiroth in the game. We know Jenova's power. We know the whole point of the reunion is to get Jenova back together. As far as I can see, there's very little to suggest you ARE chasing Sephiroth.
I still doubt the manifestation theory, creating solid mass out of will? Kinda hard to believe.
