Yes this is the hardest treasure of all to find since its the Ultima weapon aka Zidanes Legendary Weapon aka the weapon that sends your attack value sky high.

After in the beggining of disc4, u may get the Ultima Weapon(Zidane's best weapon) with ur ocean chocobo(dark one). Heres the easiest way, but remember you need to have in your possesion a Gold chocobo or ocean chocobo ( Dark Blue ). Next either fly your arse to Esto Gaza in The invincible, or with your golden chocobo, or you can swim your happy ass with your chocobo. hehe

Here comes the tricky part into finding the blasted thing. I bet everyone tried digging in the middle "exactly" where the shimmering island was located? HAHAHAHAH!!!!! IIIIINNNNGGGGG WRONG!
j/k lol. Its located on the outer rings of small ice burgs that surround the place where the island stood. Now if you look carefully, theres two circles. If you have to, go back to Esto Gaza. Its the group of rocks on the right. I think i recall after finding it, that it was across the island slightly to the right, or slightly to the left in the back. I know its hard and there is no bubbles for this weapon cause of its nature. Now how to dig it up... You're gonna need about a good 20+ peppers, so if you dont already have them startdigging them up in thechocobo lagoon, the Chocobo Air Garden, or the chocobo forrest ( which is way easier I had a beak lv 40 and one dig was all I needed to dig up anything.). Heres the easiest way to get 99 Dead Peppers. Dig up al the chocographs. Get all the treasures from the shown locations. Make sure you have a gold chocobo or this will not work! Then fly your happy ass to the upper left top corneralmost to the point where you would fly yourself to the oposite end of the map (Seasoned FF players will already know what im talking about. But can we say FF7 revisited? where we got the Knights of the Round Materia? Only on FF7 it was on the Opposite corner?)in that area you will find a small island with a forrest on it and a mountain. use a dead pepper on the crack. and walk the chocobo in. Bingo! now go talk to the fat chocobo. if you havent found every single treasure he'll babble some bullshit about having to get all treasures. heed him no ear. Once you have actually found all the treasures(including the cracks and dive sites treasures), make sure you visit him and challenge the fat chocobo to play some cards(THIS IS A MUST, i wont spoil it but trust me, THIS IS A MUST DO THING!). After talking to the big fat guy, talk to the chocobo on your left. He will offer you all his dead peppers(99 of em, so make sure when u use all ur dead peppers, come back here and get a free refill of 99 dead peppers). Gladly accept for he just saved your ass quite a bit of digging.

After you get the craps go back to where shimmering island stood (thats after you landed and are back in the water swimming). You have to dig on the outer side of the ring of rocks. Once you find it I think it takes 2 peppers to dig it up, so dig wisely.

Well, thats quite a bit of reading, but its a way of getting the best weapon for zidane. Now, if you need help on getting the other treasures to get a sweet ability for u and ur chocobo, post here.

I got all these info from ezboard.com just to let u no. Yes, it IS a forum.