Beneath a surface that once burned the eyes and seared the skin, humanity endures in a sprawling world of stone and shadow called the Breathing Depths.  The earth itself exhales through endless tunnels and skylenses, its slow rhythm pushing air through an ecosystem built on discipline and geometry.  Dozens of independent city-states cling to the walls of vast caverns where filtered sunlight spills through suspended crystal spheres of water—the Heart-Globes—that scatter pale day across terraces of fungi and steel.  Each city governs itself, bound by fragile alliances formed along political lines: monarchs with monarchs, republics with republics, traders with traders.  Their borders are fluid, their wars constant, and between them stretch the Bowels—dark, unclaimed arteries of rock that connect everything and belong to no one.  Within these depths, soldiers fight for nitrate veins and clean water, while the air itself shifts and sighs around them like a living thing.