Morj posted a theory about the One Piece being an enormous continent submerged, since there are many hints of the sea level rising in One Piece like Water 7. By erasing the Red Line, the sea level would return to what it was in the past. While I really like that theory, there are some flaws:
1 - Fish-Men and Merfolk exist. Some even live at the bottom of the ocean. That means this continent would not be a secret, but common knowledge.
2 - The logistics don't really work. If Water 7 is evidence of the sea level rising in the planet over time, then it must be happening slowly. But we got to see Jaya 400 years ago, shouldn't it be completely submerged in the current era? Assuming the Red Line covers 10% of the surface of the planet, that means the sea level would have increased by hundreds of meters in the entire world.
3 - You can't really drop the sea level everywhere in the world all of a sudden. It would basically drain almost all inhabited land in the planet, and cause the death of a large portion of the life on the planet.
I'll have to look up that theory.
1. Fish- and Merpeople being able to potentially follow the currents is a good challenge to my theory, but it's not insurmountable. Those currents could blend with known currents in a misleading way after emerging from the false ocean floor, and there can be other obstacles or anomalies that distract them.
2. I've thought about that and the slow water level change thing can easily be a red herring. Remember, at least as I'm describing it, All Blue it would have to be below the floor of the known seas. Water 7 is mainly just an example of a city on a city as symbolic foreshadowing of a double-decker world. But at the same time, some people have made compelling guesses the sinkhole of waterfalls by Enies Lobby were the results of an earlier Mother Flame incident. If so, where is all that water going so quickly that the hole doesn't fill up?
3. I don't recall suggesting there was ever a sudden drop in sea level (aside from anomalies like the sinkholes), or saying anything about sea level myself actually. Maybe that's someone else's theory? My guess was there's just more water in the ocean than people think, but if Oda goes in this general direction he would likely come up with some fantastical in-world explanation for there being breathable air, dry land,
and flowing water down there. Or he might not explain it at all, like that weird water sphere thing that Bonney got rescued from, near Egghead. (What the hell was that thing? What Jinbei said made no sense.) Science is an illusion in this series, it's pretty much magic like everything else.
