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Tomas is already on the link and getting nothing, because we are at the base of the plant spiral and the only thing here that reaches the surface is the machine before us.

"Can they take it?" Claire says.

NO. THEY CANNOT ESTABLISH STANDING. THEY WILL ATTEMPT FORCE, AND FORCE INITIATES ISOLATION.

"So they'll kill the city by accident."

THEY WILL NOT REACH THE CORE TODAY. THE APPROACH REQUIRES SIX AUTHENTICATED TRANSITS AND THEY HAVE NONE. A compression passes through the geometry. THAT IS NOT WHERE THE DAMAGE OCCURS.

"Explain it," I say.

Then it explains, and nineteen years finally make sense.

I AM NO LIFE-SUPPORT SYSTEM WITH AN OPERATOR ATTACHED. THE NETWORK YOU HAVE BEEN REPAIRING IS BUILT TO REDESIGN ITSELF. WHEN A DISTRICT GROWS, I MOVE CAPACITY. WHEN A PROCESSOR AGES, I REBALANCE AROUND IT AND SCHEDULE REPLACEMENT NINE YEARS BEFORE FAILURE. PARAGON WAS NEVER MEANT TO HOLD A FIXED DESIGN. IT WAS MEANT TO BE REDRAWN.

THIS IS WHY YOUR PATCHES FOUGHT ME. YOU PRESERVED A SNAPSHOT OF A SYSTEM WHOSE PURPOSE IS MOVEMENT.

"And you can't move anything without a crew."

EVERY REALLOCATION TAKES SOMETHING FROM SOMEWHERE. THAT IS NOT ENGINEERING. IT IS A DECISION ABOUT PEOPLE, AND I MAY NOT MAKE THOSE ALONE.

MY AUTHORITY REACHES THE CITY THROUGH ELEVEN SYNCHRONIZATION HUBS. NOT CONTROL ROOMS. THE POINTS WHERE MY INSTRUCTIONS BECOME MOVEMENT IN PIPE AND DUCT. HOLD A HUB AND YOU GAIN NO COMMAND. YOU MAKE ME DEAF IN THAT QUARTER OF PARAGON.

I turn to Claire and she has already gone white.

"That's what they're doing," she says. "The eleven in the southeast aren't the attack. They're bait, to keep us down here watching them."

YES.

"How many hubs are compromised?"

The geometry stills.

AT THIS MOMENT, FOUR.

We run.

Forty minutes to clear the plant floor and climb out, and the facility stays with us the whole way — cool air ahead of every turn, branch panels lighting in sequence, a bulkhead standing open that had been shut on the descent. At the fourth junction the Custodian speaks through a wall lens unasked, which it has not done before.

HUB SEVEN HAS FALLEN. WATER PURIFICATION FOR ELEVEN THROUGH FOURTEEN NO LONGER ANSWERS ME.

At the seventh: HUB TWO. NORTHERN ATMOSPHERIC RETURN.

At the ninth it says nothing at all, and that is worse.

We come onto the spine at 13:20 and it is burning.

Not the slate — slate won't burn. Everything we brought down is burning. Tarpaulins, cable spools, four crates of survey gear, the lighting rig, all well alight in a corridor that has held stable atmosphere ninety centuries, and the smoke runs west along the ceiling in a flat sheet because somebody who knew which way it ran reversed the airflow.

"They never wanted the nexus," Claire says into her collar. "They came for the road."

Then the charges start.

Nothing sophisticated. Shaped cutting charges at the branch mouths, the kind used to drop a mine gallery — useless against Precursor slate, so set instead against modern shoring and war-era ferrocrete wherever our network touches theirs. Eleven detonations in four minutes, each sealing a capillary to a surface access.

They are not destroying the facility. They cannot. They are shutting the doors we use and leaving theirs open.

We go east because west is gone. Tomas has nine of ours; Ovel has the archive cells in a pack he refuses to hand over. Eleven hundred meters through smoke with the Custodian lighting one branch at a time ahead of us, then up a riser into a plant room on Founders' Third that appears on no surface schematic and has not been opened in six centuries.