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Phenomena inherent to the Sub-Dimensional Plane cannot be used for mana extraction.
The Natural World is, for now, the only place where mana can be extracted.
Of all the magic stones collected from the Natural World by the wizard community, only about 10% are used by wizards to practice their Meditation Methods, while the rest are invested in maintaining the Sub-Dimensional Planes.
In other words, wizards are mana transporters.
They extract mana from the Natural World, transport it to the Sub-Dimensional Plane, and put it into the plane devices, turning it into the soil, air, vegetation, lakes, and other things of the Sub-Dimensional Plane.
Only by continuously supplying various magic stones to the Sub-Dimensional Plane can major and minor wizard forces, such as the Pure Light Court, Tomb King, Night Spirit Cult, Silent Maiden, and others, preserve their own Sub-Dimensional Planes.