The sound began as a faint, low shaking felt through the stone floor before any mortal ear could isolate its pitch.
Yan Zhiqiu was sitting upon the edge of the glow-stone berth, running her fingers lightly over the sharp-angled engravings of the Broken Slate, when her spine suddenly went rigid.
Beneath the balls of her feet, the soapstone flags seemed to tremble with a dull, rhythmic shudder—a deep, underground thrum that rose in pitch every ten heartbeats. Then, from the dark shadow of the ceiling chimney pipes eighteen arm-lengths above, a sound emerged that made the hair on the back of her neck stand on end.
It was a hollow, piercing wail.
A pure, ringing chord, shaking between eighty-eight and one hundred and seventy-six cycles, sounded through the copper air-flow angled slats. It rose like the scream of a huge brass horn. It echoed down through the upright maze shafts, filling the enclosed vault of Entry hall 003 with a shaking pressure that made the copper kettle on the kiln grate rattle against its iron trivet.
Yan Zhiqiu scrambled to her feet, her right hand snatching up the Broken Slate and pressing it defensively against her ribs.
Her eyes darted wildly toward the arched doorway of the entry hall. Her breath caught in her throat, her pupils dilating in sheer panic.
