Entry hall 439 was the main pressure-airlock that separated the heavy-duty grinding chambers of the Fourth Century from the ancient sky-high scroll halls of the Fifth.
The chamber was small, barely twenty arm-lengths square. It was built from titanic giant pillars of dense, jet-hard black rock clamped together with six-inch dogs of forged glowing-bronze. The walls were four arm-lengths thick, designed to withstand the total deep water pressure of eighty sea-weights in the event that the outer salt-crusher seawall was breached by ground-shaking shifting.
In the center of the northern wall, framing the avenue they had just crossed, sat the Keel Wedge.
It was a huge wedge-shaped gate of solid hard black rock dark rock, fourteen tons in weight, ten arm-lengths high, and three spans thick at its base. It hung suspended inside an upright stone pocket in the ceiling, angled downward at sixty degrees, held in place by two forged bronze release dogs connected to a level high-pressure trip-metal tube.
Around the outer edge of the gate opening, a deep tongue-and-groove sealing channel was lined with a molded seal ring of virgin lead and rendered seal animal fat four inches thick.
Qiao Ming wheeled the hand-barrow through the portal into the safety of Entry hall 439. He set the ash handles upon the floor flags, wiping the sweat from his forehead with his sleeve.
