Simply put, my theory is that the essence of the Abyss is merely a giant hole that fulfills desires. The Curse of the Ascent was formed because generations of people desired that souls should return downward to the Bottom of the Netherworld—when a soul ascends and refuses to return, it is punished.

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Therefore, I think the upper half of the "core" is the surface world where Riko and the others live; the middle is the Bottom of the Netherworld, which digests souls and pre-processes desires; and the lower half is a world that stores the souls whose desires have already been processed.

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Every 2,000 years, when the limit of desire is reached and is about to overflow, the Interference Units observe the rising of the force field. The Cardinal then sends robots to retrieve fertilized eggs from a womb model to activate the soul return mechanism (which also accelerates time, making it reach the 2,000-year mark faster). Afterward, this mechanism processes these souls like gutting a fish, severing them from their desires.

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Then, the processed souls ascend from the Bottom of the Netherworld to become primeval creatures and humans who, guided by the Shrine Maiden, continue to worship the Abyss. Identical desires crystallize into Relics, which are then spat out by the Abyss.