On disc 2 or 3 (easier on disc 3) take a Green/Black/Gold (doesn’t matter which one) to the second continent. Wander around the mountains between Nibelheim and Gold Saucer and you’ll easily see a waterfall in a small alcove (circular area with ground on the outside and water in the middle). Conversely, get into the submarine and head along the wall that you get to the sunken plane from until you come to another opening. Take a sharp left as you enter the opening and continue until you can’t go anymore, then surface. You’ll be in the same area.
Get off the chocobo or Submarine and chuck vincent in your party. Then enter and you’ll see some story. Exit the place and come back on disc 3 (or enter again straight away if already on disc 3) and wander up to the alter. You’ll get the final weapon and limit that you desire 😉
Amen to that