In disc 3, you can go to the Northern Crater by flying over it. There's a short scene, then you can climb down into it with a rope ladder. You can put a save point anywhere in the crater, but only one. I would reccommend saving it until you get down to the glowing pool of mako at the very bottom. Go into that pool (it's really just a hole with mako further down), & you'll have to hop from rock to rock, defeating Iron Giants & Dragon Zombies. When you get down to the bottom, you have to fight Jenova SYNTHESIS in ruins of some kind. After you defeat Jenova, the ruins will break apart, and you'll descend into the core of the planet. Once there, there is another short scene, and the fight with Bizarro Sephiroth commences. Depending on how well you did in the Jenova battle, you can use 1, 2, or even 3 parties to face him, although 1 party is usually acknowledged to be the easiest. After that is Safer Sephiroth, and an unlosable battle with Sephiroth in the Lifestream. Then, another couple of scenes, & then, congratulations on beating Final Fantasy VII.