The “Great Kingdom” was an ancient kingdom that was said to be vast, proud, and powerful. All mentions of it and its history have been erased, with even its name remaining a secret. No one knows where this kingdom was.
However, if we have divers like Mont Blanc Cricket, the Fishmen, even the pirates like Whitebeard, Roger and many more, who went down to the ocean floor. Why did nobody found even a piece of the Ancient Kingdom yet? So the Ancient Kingdom has to be hidden somewhere, where it can’t be found easily or where it is dangerous to travel to.
And the only place where this could potentially be, where it never would have been found after 800 years is the Calm Belt. Because it is just not possible for anybody to travel through it. Only the Marine is able to do this. But they dont dive. And Trafalgar never went through it, even though, he could have taken a shortcut from Northblue to the New World with his Submarine.
When Jinbe went on a journey down the ocean floor and found the Ancient Kingdom, there were Sea kings living in these places. So even for him it was dangerous traveling there.
The strange Form wouldn’t be surprising, too: The Red Line is a giant Continent that spans around the world, too. There could be another Landmass that does the same.
This may even explain the existence of the Calm Belt at all. In my head Canon, the Calm Belt is something artificial. There has to be a reason why it exists. And it could be possible, that its whole existence is based on the World government not wanting anybody to even come close.
When the Ancient Kingdom lost the war against the Twenty Kingdoms (which are now the World Government), Poseidon decided to protect the Ancient Kingdom for 800 Years, by sending sea kings.
