Okay.
"THE PROJECT OF ONE PIECE"
Now… how exactly does this project work? Will fusing 4 seas into a united ocean make Luffy’s journey worthy?
Let me explain it… It’s gonna be a bit technical so I’ll include some ilustrations.
So here is roughly how the globe looks like.
Chapter 51, page 4
The idea of “One Piece” is to create a united ocean, a perfect ocean to sail. Therefore we have two jobs : fusing all the seas into one and eliminating Grand Line abnormality. GL’s abnormality? Is that even possible?
Apparently it is possible for the AK scientists. The trick is destroy a spot where every sea on the planet (north, west, east, south, grand line, new world, and calm belt) meet in one place. There is a spot like that? Actually, there are two, and both need to be destroyed. The two spots are reverse mountain and mariejoa/fishman island.
a simple ilustration :
Think for a moment there… It made sense right? Mariejoa/fishman island also has a similar geography (minus the reverse canals).
The meeting spot are set. Next, the ancient weapons will do their job : annihilating those spots.
Ok let’s do it to reverse mountain first… FIRE IN THE HOLE!!
Even with the mighty power of the ancient weapons, the annihilation must be calculated and done correctly to get the wanted effect.
Losing a huge mass of land, The sea became even more unstable than before, a big whirlpool will appeared on the center of destruction spot, storms and tidal wave occuring everywhere. However they’re only temporary, after a couple days the raging storm shall be subsided.
And then, the magic began…
disastrous random-weather on grand line and NW suddenly doesn’t occur anymore
the magnetic field which make a normal compass useless are completely gone
Even the wind starting to blow on Calm Belt
How about sea monsters? Well, this is the time where Poseidon come to play. The Poseidon has fully taken control of the sea king and make them some kind of ‘police’ of the ocean to prevent other sea monsters from going wild. (yes I do think this is very possible)
The ideal united ocean is finally created . The project of One Piece officialy succeeded!!
But is that it???
It’s not. There’s actually one more after-effect, and it’s the most important one.
The big whirlpool that appeared after the destruction of reverse mountain, It happened because the water mass of the four seas simultaneously flowed into the center of destruction -- the place where reverse mountain was stood once. Logically, that means every kind of living things which originally lives in four seas were brought there as well. You literally could find every fish species from all over the world here.
Wait, was that sounds familiar? Exactly, the destruction spot has became that legendary mythical sea…
The United Ocean and the legendary All Blue
At least 6 of Strawhats’s dream will be accomplished because of these achievementt.
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Luffy -> he wanted to be the pirate king, “the man who has the most freedom in the ocean”. now that the ocean has been united, Luffy has literally achieved this feat.
Sanji -> it's obvious, he finally found the All Blue
Nami -> now that the entire ocean is free to sail, drawing world map is not so impossible anymore
Franky -> same reason with Nami, Sunny now can travel all over the world
Robin -> all of this happened because she found out the true history
Brook -> Ever wonder how Brook will return to the Twin Cape to meet Laboon? It seems impossible because the reverse mountain was in the way. But now, it’s GONE! They’ll meet again with ease, Bink’s Sake will be played along their emotional reunion. It will be a perfect moment.
Now how about the spot at mariejo/fishman island ??
“THE PROMISED DAY”
This mermaid… she didn’t lie at all. nor she’s wrong. She was actually foreseeing the event we’ve been talking about : the project of One Piece. And It is indeed Luffy who will trigger the event.
The ancestor of FI were actually well awared of this upcoming event.
How so? Here’s another theory : Joy Boy, he was actually the king of the AK.
Ok I know that sounds so random. There’s a theory for that but let’s save it for later…
Like a said, destroying both reverse mountain and fishman island is a must to complete the project. Therefore, King Joy Boy made a proposal of destroying FI to the former mermaid princess, Poseidon. She agreed but as an exchange she asked Joy Boy to move her people to a better place on the surface. Joy Boy gave a nod and started building Noah Arc afterwards.
Joy Boy managed to finish Noah but short after his kingdom lost a great battle against the 20 kingdoms. The promised cannot be fullfilled and he apoligized to Poseidon through poneglyph. However he still promised her “that day” would eventually come in the future, the day when fishmen and mermen live under the sun.
Conclusion : “The promised day” and madam shirley’s prediction, they are actually the same thing.
The scenario is pretty simple : at “the promised day” Luffy will trigger the attack on the destruction spot, but before that, Shirahoshi will command the seaking to move all of the FI’s citizen to a safer place (by using Noah).
Once destroyed, the same magic which happened to reverse mountain spot will also be happened here. The All Blue will appear, replacing the destruction spot. mermen and fishmen will move here later and make a brand new civilization around the Sunny Tree of Eve (if Sunny Tree has the same reputation with Adam tree, it will completely survive the destruction. There are ilustrations for this but I lost it somewhere ).
Meanwhile, this also marked the end of WG’s reign because Mariejoa --the prime symbol of WG-- was buried alongside FI in the process. Luffy finally won his war against the WG.
Related theories
Why no one could Reach Raftel? (the last polyglyph)
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"this also means I must follow the guidance the tablets I have read so far... to the ending point of Grand Line, Raftel!!"
I think Robin implied that the poneglyphs held 'the key' to Raftel. In other words, Raftel couldn't be reached by any ordinary mean. The only way to get there lies on poneglyph, and you must read all of them (it would be some sort of jigsaw puzzle if you ask me... you have to collect them all and rearrange it to get the big picture.) That's why not a single soul could reach it since Roger, because none of them are able to dechiper the texts.
so what's the deal with Raftel? What lies in there must be the final answer of the true history, as well as the trigger of "the big war" that epicbeard was talking about. well, at least that's how I see it.