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The High Thunder Gillmen Nation was structured as an enormous underwater bubble below the largest lake in Nyzzet's Labyrinth. Although this city reminds me of Aquaria for a bit, its structure is way different, and there are different techniques, technology, and magic used in its creation.
The enormous nation consists of a giant bubble made with magic, which is generated by giant Mana Generators that run with enormous Mana Crystals created through alchemy, such generators go over fifty, and they surround the entire city, generating a bubble that separates the people from the water to an extent.
Check "The Protagonist's Sister Is Actually the Strongest"!