Nanobots that were used in many different industries got out of control at some point. They was growing in the earth like mycelium. Many years ago the world was struck by enormous catastrophe. All who can be saved did it immediately. And after that they broke through to the surface trying to entangle any organic. Some of them flew into the cosmos, the second group built settlements floating in the stratosphere, others gone under water. But there were those who stayed and didn't break the conn.
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