The yellow fog did not rise; it crept.
The vapor was three times denser than common air. Because of that, it slid down the face of the fractured stonework like curdled cream, pooling in the carved runnels of the floor mosaics and filling the low drainage gut-ways with a choking, amber haze. It smelled of scorched horn, burning acid, and stale vinegar—the clear stench of potion-made aqua fortis used by the royal mining guilds to dissolve stubborn granite.
From their perch upon the narrow ledge-stone shelf fifteen deep-water lengths above the floor, Qiao Ming and Yan Zhiqiu watched the deployment through the brass sighting-vanes of the star compass. The ledge was barely eighteen inches wide, carved from a single course of dark rock headers that projected from the southern high window row. A single misstep would send a climber tumbling eighty feet down onto the broken rubble below. But Qiao Ming sat with his heels tucked beneath his thighs, his body relaxed and motionless, as comfortable upon the sheer stone face as an albatross upon a yardarm.
Four heavy iron cables now hung from the pierced ceiling vault, anchored above to unseen winch-frames anchored in the dark rock seawall. Down these cables slid two box-shaped iron cages, each carrying four tunnel diggers wrapped in jointed scale-armor of pickled horse-hide and oiled brass. Over their faces, the tunnel diggers wore heavy pig-snout respirators filled with charcoal dust and crushed limestone to stop the acidic fumes.
