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On a human colony planet that dug deep underground to escape monsters, every person is born with their umbilical cord attached to a birthgear—an emblem of the destiny they are permitted to wield. Like a newborn foal already bearing a saddle, one's social class is determined at birth. In this world, the deeper the stratum, the more noble the rank. History has been erased; no one knows how many layers of worlds once existed above them. After more than two centuries of peace, the long-gestating rebel organization "The Framework" finds the protagonist Tom—Tom, who is connected to a black cube—and the secrets of the upper levels begin to unfold.
