Entry hall 349 was the threshold of the inner world.
Here, seventy deep-water lengths below the upper scroll halls and forty deep-water lengths above the main drainage drain tubes, the stonework of the Ark underwent a dramatic, defensive shift. The wide, plastered galleries of the middle tiers ended at a giant stone wall of black hyper-dense dark rock, forty feet thick, pierced by a single opening: the Great Iron Arch.
The arch was thirty arm-lengths wide and forty arm-lengths high, its arch stones cut from single blocks of iron-stone dovetailed into the bedrock. Suspended within the upright guide-channels of the arch hung the Great Segmental Portcullis. Eighteen hundred tons of forged bog-iron alloyed with nickel and cold-hammered copper, capable of resisting the direct impact of an asteroid or the high-pressure head of eighty leagues of water.
Qiao Ming stood at the base of the eastern door post, his bare feet planted firmly upon the cold granite doorstep. The air here was dry and silent, smelling of machine grease, cold iron, and old rust.
He carried his five-pound copper hammer and a heavy iron wrench. For four hours, he had been walking the outer edge of the arch, inspecting the locking pins that held the great gate in its dropped position. The gate was down. Its lower edge, machined with a three-foot-deep tongue, was seated snugly into the corresponding groove in the bedrock threshold.
