Where is Weatheria actually? Does the WG know it exists? Kuma had to have visited it'
It's most likely in paradise, like the other destinations of the SHPs
I think they know about it's existence but they are unable to track it maybe due to the high navigation and maneuvering capabilities of weatheria.
In my Nami theory I think that Nami is the last survivor of a bloodline of true wizards from the Ancient Kingdom and prior that.
The World Government did eradicate the existence of those wizards and their related magic capabilities, also because they wanted to be the only ones remaining who will be capable to use magic, and their magic of choice that shall survive over other magic powers is the Dark Magic that Imu and the Gorosei utilize.
Nami is the only surviving mage/witch of those ancient mage bloodlines and that is why she is the only mage-type fighter in the One Piece universe, and so heavily related to wizardry and godlyness. That's why the inhabitants of weatheria look like wizards.
There is more to it, like the real life reference to the Sami People of northern history/mythology. They were witchhunted for using "weather magic" and this could explain why Nami was found on a battlefield as a baby. Her parents were clearly hunted by the World Government because of their relationships/bloodline with the ancient wizards.
That's why Weatheria is so interesting, it is one of the missing pieces to understand Nami's character and her meta in the One Piece verse.
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The Last Ark, a youtube collective of theorists, analysts and channel mods working together to crack One Piece and make content.
They already collabed with almost all known western One Piece youtubers. They have a very active discord server, they do One Piece DnD and all other kinds of stuff.