Site of the ancient Kingdom
This theory come from Emilio (french) and i find it very interesting and i have translated it, sorry if the english sound weird sometimes.
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According to the statements, the Old Kingdom was a huge country, until its destruction by the alliance that would later form the World Government. The question is: how does annihilates a country of this size? Coincidentally, we also known than antique weapons, Pluton in particular, have firepower capable of a such act. Crocodile wanted to use it to impose its power, just as the government to eradicate the era of pirates.
Continue: It is known that the use of ancient weapons could lead the world to its annihilation pure and simple as their power is enormous. It is this fact that served to justify the genocide committed in Ohara … In short, if the Old Kingdom so imposing as it seemed to be, completely disappeared, this may be due to a tidal wave caused by an ancient weapon, similar to that produced by an atomic bomb ...
It exists within the world of One Piece one and only place that seems to bear traces of this act of violence. It is Enies Lobby. There is a crater with a depth immeasurable … So of course it is possible that this crater was caused by anything else, or that there are others similar. But assume for a second that this crater is unique and is the result, the central point of the huge explosion that sent the Old Kingdom to the depths of oblivion.
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Suppose that the Old Kingdom was located in what is today the island justice. There are many strange details that should be noted and may reinforce this theory …
To begin, weather details. First, the Aqua Laguna. What could be the cause of this incessant devastating waves,? Why not this impact caused by the ancient weapon? … Although 800 years have passed, but it should not underestimate the power of fire which can ravage the world ... Furthermore, the Aqua Laguna appears headed in a proper direction, to Water Seven, and since Enies Lobby (although we note that Luffy and Co. had to pass through, front, on the road that led to Robin). This is the first item that goes in the direction of devastation caused by an ancient weapon.
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Secondly, the fact that it is constantly day on Enies Lobby. Again, we could extrapolate that the damage caused by such explosion had caused irreparable havoc. From there to cause a breach in the natural order of time and prevent the night to fall again at that specific location? Why not. The fact is that it is difficult to not report the existence of the mysterious crater and weather anomaly in the same place (and a tidal waves heading in the opposite direction of the island - Enies Lobby, contrary to Water Seven, would not have withstood the onslaught of killer waves after so many years, for the simple fact that buildings are built low over the ocean).
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Both first details reinforce the idea of an attack on the ancient weapon, and always assuming that this attack was the one who buried the Old Kingdom, it would be interesting to watch the nearby islands and to try to find any relationship with the now extinct giant country.
Nearby, Water Seven. A city that has seen its slums engulfed by the repeated assaults of the Aqua Laguna, which formerly was much more impressive. We notice it when the waves retreat and the lower town discovers … Water Seven, part of the Old Kingdom? Again, why not. It is close enough to Enies Lobby, and most importantly, we know that an ancient weapon has been manufactured in the past … The location of these weapons is shown on the Poneglyphes, and Poneglyphes is from the Old Kingdom, the link is easy to do. Of course, we can always say that Water Seven was a subsidiary and did not necessarily fully part of the ancient Kingdom
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Yet, the fact is that all around Enies Lobby, the islands are many and not very remote, since all of them are connected by the sea train. Remains of a country gone? … In any case, this proximity has never been seen outside this area, where travel from island to island are much longer. Sometimes near extreme it seems, because the CP9, after his defeat at Enies Lobby, followed the railway to St Poplar in walking.
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Strange coincidence … But speaking of chance, there is another even more striking: how to explain that the judicial seat of government, the headquarters of the Navy and Impel Down (the largest prison in the world) are located in the same area all at once?
Reflecting on how the government has hidden all traces of the Old Kingdom, erased their existence and finally took their place in history, it is possible that this desire to "crash" has been pushed up to the extreme, the government decided to sit where once sat their greatest enemy. So who could have suspected anything? Who could doubt than existed formerly a kingdom of such magnitude while there is now the greatest world powers, as they always had been there somehow …
This may well stick with what we already know of a government that has not hesitated to destroy a country for political reasons at least questionable..
And nearby, Mariejoa, very top of Red Line at the top of the world... As a divine presence (the "holy land") who admire his domination over a world that she has forged with his hands after having destroyed…
About Impel down now:
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Even in the world of One Piece, is it possible to build a Prison under the sea, located in calm belt, surrounding by Sea monsters?
Personally, I doubt that it is possible.
This Prison is suspicious if we take into consideration what has been said in the theory. If it's a part of the AK, then maybe that this edifice sank into the ocean or has been engulfed. This is possible, regarding of what happened with the golden bell of Skypiea.
We can also take this point of view from the WG: "What can we (WG) do about this building, which is under the sea? Lets do a prison, it will be the hardest prison to infiltrate or escape."