The completion of the Deep-Rail had synchronized the mountain's industrial output, but it had also created a rhythmic, mechanical signature that pulsed through the limestone. To the sensitive seismic tools of Arch-Magister Vane's remaining surface scouts, the mountain was no longer an impenetrable silence; it was a humming machine. Every ore car that gripped the cable, every roller that spun in its housing, sent a vibration upward. Kael knew that if the Empire could map the tunnels by their sound, they could pinpoint the residential vaults for a targeted surface-breach. He initiated the Silent Pulse—a project to acoustically decouple the industrial grid from the mountain's bedrock.
The technical challenge was the management of vibration-transfer. Steel rails bolted directly to limestone acted as a massive acoustic amplifier. To solve this, Kael engineered Composite Rail-Ties. He utilized a sandwich of materials: a base layer of compressed peat-fiber for bulk, a middle layer of lead-sheet for high-mass damping, and a top layer of rubberized sap harvested from the southern bog-vines. This "Suspension Bed" was designed to absorb the high-frequency "chatter" of the wheels before it could reach the stone.
The grit of the installation required the Deep-Rail to be rebuilt while in operation. Drax and the Tier 6 engineers worked in the four-hour "Cooling Shifts," lifting sections of the live rail-line with hydraulic jacks and sliding the new composite ties into place. They had to work in the narrow gap between the moving ore cars and the tunnel walls, their movements timed to the relentless pulse of the transport cable.
Kael augmented this physical damping with the principle of Destructive Interference. He utilized the Geothermal Bore's excess steam to power a series of "Counter-Vibrators" positioned along the primary limestone ridges. These machines were tuned to emit a frequency exactly opposite to the mechanical hum of the Deep-Rail. On the surface, the two waves would meet and cancel each other out, leaving a "dead zone" of silence that masked the subterranean activity.
Socially, the Silent Pulse brought a strange, heavy stillness to the residential tiers. The rhythmic heartbeat that the citizens had grown accustomed to was suddenly gone, replaced by an artificial silence that some found unsettling. To help the population adjust, Kael used the acoustic pipes to broadcast a very low-volume "Nature Wash"—the recorded sound of the River Ash and the wind in the salt-spurs. It was a sensory anchor that prevented the psychological "pressure" of total silence.
A material failure occurred in the rubberized sap compound. Under the constant friction and heat of the east-reach curve, the sap began to polymerize and turn brittle, losing its damping properties. The seismic mirrors in the command vault showed a sudden, jagged spike in the surface-vibration—the "Silent Pulse" was failing in the most critical sector.
Kael utilized the Graphite-Lubricant Infusion. He realized that the rubber alone couldn't handle the friction. He ordered the "Scrap Reclaimers" to process the graphite electrodes from the old galvanic cells into a fine powder. This powder was mixed with the sap, creating a self-lubricating composite that stayed pliable even under extreme heat. The "hiss" of the water-cooled rollers was now muffled by the soft, greasy cushion of the graphite-sap.
The population count climbed to 790 as the first "Surface Scouts" from the northern baronies arrived. These weren't refugees; they were skilled stonemasons and carpenters who had heard of the "Great Vaults" and wanted to join the expansion. They brought with them the expertise needed to turn the raw limestone galleries into finished architectural spaces.
Kael stood in the East Reach tunnel, resting his hand on a rail-tie as a ten-car train glided past. He felt almost nothing—only a faint, rhythmic tick that disappeared into the floor. The mountain was silent again.
The pulse is hidden, Elms, Kael said, observing the Mirror-Readers' calm displays. We are invisible to their ears now. But as the population reaches eight hundred, we are facing a new kind of "Noise"—the biological waste of a city. Our current lye-scrubbers and water-filters are reaching their limit. We need to expand the aquaculture tiers into a full-scale Hydroponic Ring. We need to turn our waste into oxygen and food on a massive scale.
Kael began the designs for Tier 7: The Green Ring—a circular gallery that would wrap around the primary geothermal bore, utilizing its light and heat to grow the crops needed to sustain the next thousand citizens.