The seismic needles had revealed the Duke's endgame. By tunneling beneath the Sinking Trestle, the "Ghost Engineer" was bypasssing the high-pressure water monitors and the galvanic alarms. They were aiming for the "root" of Ashfall's logistics—the timber piles that held the Iron Road above the northern marsh. Kael knew that if the sappers reached the trestle supports, they could use their sulfur-based charges to liquefy the soil, causing the bridge to collapse under the weight of the next ore shipment.
Kael could not wait for them to surface. He initiated the Counter-Sap Protocol. This was not an industrial project, but a surgical, subterranean interception. He selected a team of six: himself, Drax, and four Tier 0 laborers who had spent months in the deep pits of Outpost Alpha. They would dig a narrow, reinforced interceptor tunnel from the edge of the limestone ridge, aiming to cut across the Duke's path before they reached the bridge.
The "grit" of the counter-sap was the environment. The soil beneath the marsh was a treacherous mix of silty clay and pressurized pockets of methane. As they dug, the tunnel walls bled black, foul-smelling water. Kael mandated the use of Segmented Shoring—iron-shod timber frames that were hammered into place every three feet to prevent a cave-in.
To ensure they were on target, Kael used a Subterranean Geophone. This was a simple but effective device: a hollow copper cone pressed against the tunnel face, connected to a leather tube. By listening through the tube, Kael could hear the "clink-thud" of the Duke's shovels getting louder. The sound didn't travel through the air; it vibrated through the very bones of the earth.
"They're close," Kael whispered, the air in the tunnel becoming thick and hot. "Less than ten feet. We need to brace for a Pressure Breach."
Because the Duke's tunnel was likely deeper and closer to the water table, there was a high risk that breaking into their shaft would cause a sudden inundation of marsh water. Kael prepared a Sluice-Shield—a heavy iron plate mounted on a pivot at the front of their digging face. If the water surged, they could slam the shield shut, sealing their tunnel and preventing a total drown-out.
The social atmosphere in the tunnel was one of claustrophobic solidarity. Drax and the Tier 0 men worked in a rhythmic, silent trance. In the darkness, the hierarchy of "Baron" and "Laborer" dissolved. They were simply organisms fighting for oxygen in a world of crushing weight. Kael handled the geophone and the "Chemical Sniffer"—a small cage containing a reactive lime-paste that would turn gray in the presence of lethal marsh gas.
The breach happened with a sudden, wet slap of a shovel. The earth at the face of Kael's tunnel simply gave way, spilling into a larger, more finished shaft. The light from a sputtering sulfur-lamp spilled in from the other side.
The Duke's sappers were not ragged conscripts. They were professional miners, equipped with leather respirators and specialized iron picks. For a moment, both teams froze, staring at each other in the dim, orange light of the tunnel.
The combat was a brutal, short-range affair of "Industrial Violence." There was no room for pikes or bows. The men fought with short-handled mining picks and heavy iron pry-bars. Drax surged forward, his massive frame filling the narrow breach, using his pick to disable the Duke's lead sapper before he could ignite a "Friction-Fuse" on a nearby sulfur charge.
Kael didn't join the melee; he focused on the Atmospheric Variable. He noticed the Duke's sulfur-lamp flickering blue—a sign of rising methane levels. "Drax! Back! The gas is peaking!"
Kael triggered the Pneumatic Bellows. He had a pressurized air-line running from the surface, intended for breathing. He reversed the valve, turning it into a high-pressure "Air-Cannon." The sudden blast of oxygen-rich air into the Duke's methane-filled tunnel didn't just clear the lungs; it disrupted the "explosive ratio" of the gas, making it impossible for the Duke's fuses to catch.
The Duke's sappers, panicked by the sudden roar of the air-line and the ferocity of the Tier 0 counter-attack, scrambled back toward their own entrance. They left behind three heavy sulfur-charges and a detailed map of the trestle's subterranean piles. Kael and his team sat in the captured tunnel, their faces caked in black silt. They had stopped the sabotage, but the breach had exposed the Sinking Trestle to a new danger. The Duke's tunnel had significantly weakened the soil density around the central piles.
"They didn't blow it up, Elms," Kael said, his voice a hoarse rasp as he examined the Duke's charges. "But they've hollowed out the ground. The next time the Axle Train crosses, the piles will have nothing to grip but air. We've saved the bridge from fire, but we've lost the foundation to the void."
Kael looked at the sulfur charges. He didn't dismantle them. He began to rewire the "Friction-Fuses" to his own Galvanic Line.
"If they want a hole under my bridge, I'll give them one. We're going to turn their sabotage into a Grouting Explosion. We're going to use these charges to collapse the tunnel and 'flash-bake' the clay into a solid stone plug."