Entry hall 140 was known in the ancient land-mapper ledgers as the Hall of the Balanced Stone floor.
Spanning three hundred paces square beneath a coffered dome seventy arm-lengths high, the room was paved with one hundred and forty-four massive square slabs of dark stone, each slab ten paces on a side and three arm-lengths thick. To the naked eye, the floor looked like an ordinary, unbroken expanse of massive stonework. But beneath the stone lay a marvel of ancient high-pressure building: each dark rock slab was supported upon a central bronze piston that floated inside an underground metal tube filled with heavy stone salt water, connected to a fine, detailed web of lateral high-pressure balance-pipes.
When the valves in the lower control gallery were shifted, the slabs did not merely tilt. They slid. Driven by the water-pressure head of the incoming and outgoing tides, the massive stone squares could glide horizontally along lubricated bronze rails, shifting their positions in complex, non-Euclidean sliding-block patterns that transformed the wide, open hall into a shifting maze of deep trenches, isolated stone stands, and dead-end cul-de-sacs.
Into this deceptive chamber marched the final assault reserve of the Obsidian Scroll hall. Three hundred master tunnel diggers clad in heavy, said clearlyvoid-iron armor, with forty heavy mining drills driven by high-pressure air-powered engines.
