The entrance to the inner chambers of the Grand Scroll hall was an ancient underwater high-pressure portal known as the Siphon of the Scribes, rather than a conventional dry hall.
To preserve millions of hand-copied silk scrolls, pressed bamboo tablets, and pale limestone water-charts from the decay of underground damp and the predation of mold, the First Patriarchs had engineered the archives as a series of isolated sky vaults. It was sealed from the outer world by inverted U-tube water-traps.
A traveler wishing to enter the storehouse had to go down three arm-lengths beneath the surface of a calm marble pool, walk four paces through an underwater archway of polished red stone, and emerge on the other side into a fully dry, hermetically sealed air where the air had remained undisturbed for eight hundred years.
The stone-built design of this barrier was deceptively simple yet fully infallible. Because air is compressible and water is essentially incompressible, the volume of trapped air inside the dome acted as an elastic air-powered plug. Even if the outer basin rose by twenty deep-water lengths, the water level inside the drain tube would only advance by three inches before the rising counter-pressure arrested the ingress.
