With the Master Siphon Gate fully open and the war-cleaver discarded into the deep, the Grand Central Round hall entered its final high-pressure phase.
The water level, which had reached twenty-eight deep-water lengths at the height of the deluge, was now dropping at the rate of two arm-lengths a minute. Eighty thousand cubic arm-lengths of water were swallowed every sixty seconds by the underground water chute. Then, the central vortex sped up into a terrifying water-flow whirlpool.
The Abyssal Drain was the master deep-rock chimney of the entire mountain range. Plunging vertically through six hundred arm-lengths of solid granite, its smooth circular walls had been scoured by millions of years of underground runoff before the First Patriarchs ever set foot in the hall. At its bottom, the upright shaft connected to the Primordial Aquifer. A boundless underground sea that circulated beneath the earth-shaking plates of the Middle Kingdom, absorbing the drainage of four provinces and returning the moisture to the air through deep fiery vents.
The spinning funnel of dark water had widened to sixty arm-lengths across. It dropped thirty deep-water lengths below the surface in an inverted cone of roaring, crystal-clear foam. At the bottom of that roaring throat lay the master threshold of the Abyssal Drain—an unplumbed upright shaft six hundred arm-lengths deep that pierced the bedrock of the mountain, discharging the deluge directly into the underground water beds of the Great Flooding.
