Quick theory about what the World Government did during the Void Century. In summary they flooded the world to kill a large part of the population and separated the seas to prevent unification against them.
So why a flood? There are several reasons:
- The Noah. This is pretty weak evidence, but Oda seems pretty well versed in Biblical lore (Devil Fruits, Adam Tree, Eve Tree) and in the Bible, when God decided humanity was too wicked, he flooded the Earth and started over with Noah’s family. The presence of an ark in the story suggests to me that at some point a flood happened and it was caused by the Gods aka the Celestial Dragons.
- It’s called the Great Cleansing. This could just be metaphorical, but water is typically used for cleaning. If you’re going to “clean” the world, water is a very literal way of doing that.
- Imu backwards is Umi, the word for ocean in Japanese. His name suggests he has water powers just like Eneru’s name suggests energy powers. Assuming he is the one who does the cleansing, a flood makes sense (I also believe that he created the Calm Belt using his Awakening to split the seas further).
- The Celestial Dragons decided to move to the top of the Red Line. Like the Nefertari family, the Celestial Dragons all had kingdoms they ruled, but at some point they moved. They had vast land to control by themselves, why all move to the highest point on Earth with no resources and shared control? It seems plausible that the Holy Land is the one place the flood waters can’t reach. ‘Holy’ because the sea devil can’t reach it. The Holy Land is kind of a doomsday shelter.
- The stupid bubble helmets. When they aren’t in the Holy Land the Celestial Dragons are always wearing those silly bubble helmets. They claim that it’s to avoid breathing the same air as everyone, but we know better. They ride slaves and take commoner wives, proximity to commoners isn’t an issue. Oda has repeatedly shown those bubbles are used primarily for breathing underwater. If there was a giant flood at some point, a group of people paranoid enough to move to the Red Line might also be paranoid enough to always wear a scuba suit when they leave the Holy Land.
*Theory by EndCentury