Entry hall 204 was a stone-built paradox born of ancient Daoist shape.
Viewed from the eastern portal, the chamber appeared to be a conventional barrel-vaulted nave, eighty paces long and thirty arm-lengths wide, lined with twin rows of fluted red stone columns. But to a land-mapper who understood the mathematics of the Second Epoch, the nave was a sight and sound illusion. Its ceiling was cut on an elliptical cross-axis whose two exact foci were separated by sixty paces of open air.
If a man stood at the eastern focus and dropped a copper coin upon the flat stone slabs, the sound would not radiate outward in a normal sphere. The curved red stone vaulting would gather the sound waves, reflecting them through the air to converge with pinpoint clarity directly at the western focus. There, the sound would ring as loud as if the coin had dropped at the listener’s ear.
Qiao Ming stood at the eastern focus, his bare toes resting upon the bronze center-pin embedded in the pale limestone floor.
In his hand, he held a small, polished sphere of cast sea-bronze, three inches in width, punched holes in with eight circular sound whistles.
This was a Phantom Sounder—an ancient water-mapping fine-tuning tool used to map the sound boundaries of flooded caverns. Suspended in an air draft or swung on a line. Then, the whistles produced an uncanny, multi-tonal drone that sounded exactly like the heavy, rhythmic splashing of ten armed men wading through knee-deep water.
