I want to put forward a theory I thought of while going through One Piece details with friends/family for the hundredth time. We know the following information to be confirmed true:
Enel's Great Space Operation
The moon does, and did, have life on it
there is at least some atmosphere there as the small robots could fly up using balloons and Enel could make it there on his ship the Maxim
Also supporting the atmosphere theory is in Oda's cover art drawings there is a gradient to the sky instead of just black, implying something is reflecting light back, aka atmosphere
There is an ancient civilization that both the Shandians and the mechanized robots they created inhabited until they ran out of resources
Impel Down, Marineford, Enies Lobby location:
There is a location in the world that is eternally day/afternoon, Enies Lobby is referred to as afternoon afternoon island, while there is a hole under the island, as it is not referred to as "day day island" it would not be at the axis of a planet, but near it, counting out the hole under Enies Lobby for this theory.
The three are located around a large whirl pool
Those above things tell us something, assuming the world is a globe, which I believe we can assume from the Ohara planet representation, would mean that Enies Lobby and the area is right next to an axis of the planet. One of the axis pointed close to directly towards the sun.
This also means that the other axis of the world is eternally night and would probably be a very cold or hard place for life to be.
The three (marine locations stated above) are located in the Paradise section of the Grand Line, the large whirl pool close to the Calm Belt as Impel Down is in it and part of the three locations. I don't think it would be a stretch to say that that whirlpool is probably the, if not very close to be, the axis of the world.
This is where my theory comes in, I believe the other side, is both in the new world, and either also has a whirlpool or other large scale phenomenon. This other axis of the world, is pointing towards the moon.
With the above information in mind, I posit that at some point, the ancient weapon Uranus will be found to be located at the moon, was fired in the past void century, and created a massive hole in the planet, realigning it's axis to face the sun and the moon and creating these enormous oceanic phenomenon. From the name Uranus we know it can be associated with "Sky" or "Heaven" which I believe further gives evidence to the moon being where Uranus is, and the world's axis being so close to large scale oceanic phenomenon being proof that it was fired at some point.
The other part to this theory does not have much evidence, it involves the Paramount War. At the beginning of this, somehow, White beard gets his flagship and his fleet in to Marineford undetected. My theory for this, using the primary theory as evidence, would be the following:
These things are found in issue 551:
A pirate fleet gathers at Marineford, somewhere that should be protected by the whirlpool and special current but a fleet got there anyway without being noticed.
White beard and his four primary ships appear in the center of Marineford from underwater, showing that they are coated with a substance similar to, if not the same as Shabaody Archipelago's ship coating.
The last place we see White Beard in issue 433 is shown to be in the New World.
I believe it is safe to assume that White Beard had not traveled back to Paradise from the New World between the time that passed between issues 433 and 551. With that I would put forward that White Beards ships, at least the four of that we knew went underwater, traveled through the planet appearing inside the whirlpool located at Paradise, and made their way to Marineford underwater.
Could that be a bit of a stretch? Yes, they could have simply gone underwater close to the three locations and traveled there. It does not sound plausible to me however that the entire Marine force, who keeps sharp eyes on the Yonko, would not know that White Beard had traveled back to Paradise and was on his way to the execution grounds.
Let me know what you all think this is just a theory that came to mind that I think sounds plausible for Oda.