Spoilers Chapter 1113

Notare che fra gli abitanti di Baldimore gira voce che Franky abbia trasformato sé stesso in una nave

C'erano disegni di Oda in cui Franky in un futuro alternativo mandava talmente oltre le sue costruzioni di navi da battaglia da diventare lui stesso una nave da battaglia.

It was talked about when they introduced the ark Noah during Fisherman Island arc, and also when they revealed current Wano to actually just be the Fuji mountain top with the original Wano fully submerged.

The theory goes that the world was originally land that was at Fish-man Island's Fish-man district altitude, because the buildings were the same as the ones shown in Oda's one-shot 'Monster' that featured Ryuma from 800 years ago(kinda farfetch imo). Plus nobody ever knew how it came to be, as confirmed bu Neptune when he said it already existed long before Fish-man shunned from the main districts took over.

Fish-man and Mermans were the ones who adapted and evolved to survive in the sea during the first great flood with the help of Joy Boy, while those who didn't was because they chose to find land instead(also with the help of Joy Boy). Joy Boy knew the next person to inherit his will and name, would go on to cause another war just from reappearing, since WG will not tolerate his existence.

The previous flood was the result from the war between the unnamed Great Kingdom and the 20 kingdoms which formed the World Government after they won, everything from raising the Redline to Wano's closing was from Joy Boy's precautionary preparation in the event he lost the war(which he did). Lunarians were suppose to protect the Redline while waiting for Joy Boy's return but they were destroyed, Buccaneers also had a role but it still hasn't been revealed, Wano's job is to protect Pluton until Joy Boy returns, Elbaf's role has yet to be fully revealed aside from waiting for Joy Boy to return.

Sky Islands were also theorized to be survivors from the Great Kingdom who used their technology to turn clouds into habitable islands, allowing them to stay outta reach from both the flood and WG. The moon was the most talked about part of the theory, as the moon controls the ebb and flow of oceans across the world. The theory is that the moon was formed when Joy Boy throw the entire Great Kingdom out into space to protect it from WG once he realize the war was lost. The automatons that Enel found during his cover story were the caretakers awaiting for Joy Boy's return. That explains why Enel was worshipped mistakenly, since Joy Boy was the only one that could have survived the vacuum with his ridiculous power, and he also creates lightning whereever he goes(conqueror haki lightning) and ability to grab and wield lightning