Two notes beat against each other and the concrete starts to listen.
"Source moved," Mia says, tight. "New signature under the central span. Not ours. Offset frequency."
"Offset how?" Juno asks, already turning.
"Two metronomes out of sync," Mia says. "You can lock them, but if they drift—"
"They fight," Ariad finishes. She's already moving, feet careful, eyes on concrete. "There's a second system."
Rao swears softly—the first crack in his calm. We sprint with him. The conduit bends with the river curve; the tunnel widens; the hum lurches. This lattice is wrong. Not the three-one-three we know. Double notch, then four, then none. Either ignorance or malice.
"Not ours," the hammer woman says, breath fast. "Not city. Not us."
"The Board?" I ask.
Ariad's look: the Board doesn't do clever without a contract.
Cracks spider-line the slab. "Brace now," Ariad orders. "Kanon, Chalk. Big X. Juno, hands on metal—if the torque lies, give it Ken."
Rao's man rips a brace gun from a roll we didn't see. He's fast; he knows where ugly things hold long enough to think. Juno palms bolts, Ken-listens for stripped threads; torque, shove, set. I paint a thick Kein Chalk X across the worst panel. It anchors nothing, but it tells feet where to be and hands where not to touch. Sometimes that's the difference between a bruise and a body.
"Boat making another pass," Mia warns. "Board in ten. I can't blind both."
"Blind the Board," Rao says, and grins when Juno glares. "Joke." A beat. "Mostly."
Ariad slams the Anchor Gun at a new angle. It kicks; the spike bites. Clip. Clip. Her hands shake; she doesn't stop. "Hold," she says—at the wall, not at us—and the wall actually listens.
The two hums drift, collide, settle half a step closer. The beat slows. Good lattice dims under our braces; the wrong pattern yanks bright where it pulls. Someone installed a system to fight the first. Idiocy or sabotage—effect's the same.
"Whoever did Pattern B wanted Pattern A to fail under load," Ariad says, diagnosis pure. "Sabotage."
"Not ours," Rao repeats.
Juno doesn't let him off. "You keep saying that."
"I like truth," he says.
A gel round pops at the mouth, splashing a slab near our good work. It meanders toward a brace spike like it knows where to hurt. I hiss Phantom Steel left; tube man takes the bait, fires off-line. The round dies in a dead zone—still sizzles, doesn't eat our anchor.
Boots slap behind us. Board windbreakers flood the tunnel. Kade in front, coat the wrong color for every place we go. His eyes drink Chalk, lines, gray-jackets standing, and our braces. He doesn't blink at Rao.
"What," he says, flat, "did you do."
"Kept your conduit from folding," Ariad says. Sweetness not included. "Also found a second pattern fighting the first."
Kade clocks the double-notch spikes, files it under ugly. "Chain everything," he tells his team. To Juno: "Zone tag filed. Good."
"Gel crew at the mouth," Juno says. "Box them."
Half his team moves. The rest films. Rao stands where the cameras like him. He probably wants the record.
Kade looks at my Chalk X, then at me. "Erase when you're done."
"It keeps feet right," I say.
"It also shows saboteurs where to cut," he says.
"Leave it," Ariad says. "Until I say erase."
Kade weighs an argument. The hum shifts ugly underfoot and the wall groans like an old man in bad weather. He swallows it. "Fine."
Mia pings my visor. Unknown number. I mean to ignore it. I don't.
A single still: the wrong-pattern spikes circled, notch code underlined. Text: YOU'RE CLOSE. NOT MY HAND. DON'T PULL IT YET. WALK IT PAST THE PYLON WITH THE SCRATCH.
"Zann?" Mia breathes, reading my face.
"Probably," I say.
Juno catches my jaw at work. "What."
"Instruction," I say. "Don't pull the wrong spikes yet. Walk load past a scratched pylon."
Ariad doesn't ask who sent it. She just asks, "Where's a scratched pylon in a concrete tunnel?"
Rao points with his chin. "Not in here."
We move because the tide won't wait. Outside, at the mouth, one river pylon wears a long gouge down its side where a barge kissed it last year. Underwater, lattice wraps it heavy. The wrong pattern hooks it light.
"Walk it past," Ariad says, thinking out loud. "Okay. Okay."
Anchor Gun. Fire. Set a high line. Rao's man clips; hammer woman ties, quick. Juno braces a bolt with her palm, Ken through her wrist, reading the thread like braille. I draw a big Kein Chalk arc like a coach's play across the slab, the path we want force to take—lie on concrete, truth for hands.
"Now," Ariad says.
We pull, tighten, release in order. Load creeps along the blue arc, away from the scratch. Wrong pattern fades as stress leaves. Good lattice takes it. The beat between hums slows again.
"Hold," Ariad says.
The river exhales. The wall stops buzzing our boots. The conduit settles into the new map.
Kade watches Ariad like she's a field surgeon doing miracles in a tent. Respect slips; it makes him angry to feel it. He swallows.
"Chain of custody," he says to remind himself who he is. "Samples of both patterns; video of all brace notches. Names," he adds to Rao, like the word is a cuff.
"Rao," Rao says.
"Last name," Kade says.
"Rao," Rao repeats. I hate that I like the joke.
Hammer woman slips close to Ariad, quiet: "They'll stop you."
"They can file a form," Ariad says, and some fear leaves my shoulders.
"Board in the mouth," Mia reports. "Gel crew face-down. Same men."
"Pack," Juno orders. "Tide."
We move fast. Minnow reels in. I wipe Chalk Xs where they might guide a saboteur; leave the big arrows that tell tomorrow's crew where to stand safe. Kade notices me making the choice and pretends he didn't.
Rao drifts toward the mouth. I block on instinct. "Who's running the wrong pattern?"
He weighs the give. "The city loves contractors," he says. "Contractors love margins. Margins love shortcuts. Or someone wants Pattern A to fail so you yell at us while they do what they want in a different hole."
"That wasn't an answer," I say.
"No," he says, friendly.
"Zann sent the text?" I ask, because caution is taking a nap.
He glances at my badge name. "You'll catch him when you stop assuming he's the only one writing."
That lands like a pebble in a shoe.
My visor blinks: Board vans inbound—and my phone buzzes again, a signature I haven't seen in months: —Z.