I don't stop.

  The crystal comes free in a surge of black water and gray matter. It's smaller than I expected. Maybe the size of my fist and it's trying to escape. Squirming in the bristles like a fish in a net, pulsing with that faint inner light, fighting to get back to the safety of the filth.

  But I’ll never let it.

  Grabbing it instead I slam it against the pipe wall before it can slip free. Pin it there with my hand.

  It writhes, pulses. The sludge around my legs surges onto my arms, trying to climb, trying to reach.

  The shamblers are coming. Splashing through the chamber. Seconds away.

  I flip my knife. Icepick grip. Blade down.

  First strike. The crystal's surface resists, harder than it looks, that faint glow intensifying with each impact. The evolutions, they’ve made everything about it worse. My arm jars from wrist to shoulder.

  A shambler reaches the pipe entrance. Bloated hands grab at my pack, trying to pull me away from the wall.

  Second strike. I feel something give, a tiny fracture. A weak point forming.

  I wrench sideways, throwing the shambler off balance. It stumbles into the sludge which is around my chest now, of course it is. Nothing down here fights fair. Another takes its place immediately, fingers closing around my knife arm.

  Third strike. The fracture spreads. Compound damage. A divot forms on the crystal's surface… small, barely deep enough to catch the blade's point, but there.

  Another shambler yanks my arm. My next strike goes wide, scrapes across the crystal's surface without biting. The sludge reaches my neck. My vision grays at the edges. The psychic screaming fills my skull until I can't tell which sounds are real. The crystal pulsing faster, brighter, like a heart beating in panic.

  But I don’t give in. I’ll never give in.

  My fingers are cramping around the knife handle. I've been gripping too hard for too long. Still I drive my elbow back. Feel it connect with something soft. The grip on my arm loosens for half a second.

  It’s enough.

  My whole body leans in. Everything I have left. My vision is almost gone, the air in my lungs nearly spent. Only me, the blade, the crack and my will behind it. But I don’t care about the screaming in my mind because I will never give in.

  Millimeters. That's all the ability gives me. Barely enough to notice under normal circumstances.

  But millimeters delivered instantaneously, with System enhanced force behind them, at exactly the right moment. It becomes something else entirely.

  The dagger punches through.

  The crystal shatters into three chunks. I catch them by suffering through the impact and keeping them trapped against the wall. Did my constitution help with this or maybe it was simply my greed? It was hard to tell the difference anymore.

  The effect is immediate. The shambler on my arm goes limp, not dead, merely empty, like a puppet with cut strings. It collapses into the water and in the chamber beyond, I hear a cascade of splashes as twenty bodies drop in unison.

  The sludge stops moving too. It slides away.

  The pressure in my skull vanishes.

  Silence.

  I stay in the pipe for five seconds. Breathing hard. Waiting for something else to move.

  Nothing does.

  I dismiss the notifications with a thought. Breathing it all in can wait. The doctor, Rios, Sofia can't.

  Removing my hand from the wall I pocket the three chunks of crystal in the front pouch of my kevlar. Something about them pulses against my palm. Latent energy where there shouldn't be any. I'll figure out what they're worth later.

  Turning I wade back into the chamber. The shamblers float face-down in the water, I recount them, thirty-six of them, two more arrived after. I count a third time as I push through. Thirty-six people who went into the sewers and never came back. Thirty-six bodies the Grudge collected, puppeted, used.

  Thirty-six people I'm responsible for. Four times I had this thing wounded. Four times I chose caution over commitment. Four times I choose Lily’s survival over the slaying of this monster.

  This is the interest rate on "later." The cost of everyone else around me.

  I find Sofia on her back at the center of the chamber. Her lower half submerged. Shamblers collapsed across her legs and torso, dead weight now instead of grasping hands.

  Somehow her face is above the waterline.

  Her eyes are open. Staring at nothing. Her lips are moving but no sound comes out.

  “Doctor”, I pause. “Rios."

  No response. Her pupils are dilated, unfocused. Shock, maybe. Oxygen deprivation, more likely. Or something else?

  I check her pulse, rapid but strong. Breathing shallow and weak. Eighty-seven seconds under that pile. Seven seconds inside the calculated margin but she's alive. Conscious, technically, but not tracking.

  "Sofia. Can you hear me?"

  Her eyes drift toward my voice. Through me, past me. Looking at something that isn't there.

  "Lights," she murmurs. "There were... the lights said..."

  Delirious, sure. But verbal. Verbal is a positive indicator.

  Then I see it. A faint glow near her eyes, the telltale shimmer of a System notification she hasn't dismissed. I can't read it, notifications are private after all but I recognize the color. The particular shade of blue that means advancement.

  She leveled. Perhaps more than once.

  The fight. The shamblers. Even being buried under them, even being used as bait, even nearly drowning, the System counted it as participation. Awarded her experience for survival. If she leveled enough, the stat points could make the whole recovery process a distant memory.

  "Sofia. What level are you?"

  "The lights," she says again. "They're so..."

  "Your level. What does the notification say?"

  Her eyes focus, barely.

  "Six." Her voice is distant, dreaming.

  Level six. The System did exactly what all of us needed it to do. Strengthen ourselves, even if it's only with participation XP. Four minutes of terror bought her two levels, maybe three, could easily be close to Level 7.

  I exhale.

  Her body is already improving, damage resistance increasing, baseline improving along with a thousand inconceivable upgrades that separate the survivors from the corpses on the street above.

  The System, a curse and boon. Yet she's not dead. The pile of shamblers would have crushed baseline ribs, collapsed baseline lungs, would have drowned a baseline human. But her body, even slightly enhanced, was enough, surviving until the experience hit, burning through the worst of the damage, keeping her breathing when biology or willpower alone might have quit.

  The System wanted us to try. This much I understood about it.

  "Can you stand?"

  She blinks. The focus comes and goes.

  "I don't... everything feels..."

  "Alive, I know. It gets easier with each level. " I pull her arm across my shoulders, haul her upright. She sways but stays vertical. "We need to move."

  "What about them..." she gestures at the bodies floating around us.

  "Can't help them. They were dead long before we got here. The Grudge just... borrowed what was left. Now they can rest."

  The scavengers will take care of this.

  She looks at the floating corpses. The maintenance uniforms with their GW Memorial logo sewn on. The bloated faces of people she might have known, might have treated, might have passed in hallways before the world ended three weeks ago.

  "This is what was down here," she says. Still slurred, still distant. But the accusation woven through is clear enough. "You knew."

  "Yes."

  I don't explain it further, I don't tell her about the hive mind evolution, or how it learned extrapolation from every encounter, or why I couldn't risk naming it out loud down there. I guide her instead toward the ladder.

  "Climb," I say. "I'm right behind you."

  She looks at the ladder, then looks at me. Something moves behind her eyes, a question she's not ready to ask, a conclusion she's not ready to reach.

  Then she starts climbing.

  Her movements are clumsy at first. The System enhancement is too new, her body not yet calibrated to its own improved baseline. She overshoots the first rung, nearly loses her grip, has to pause and breathe.

  But she doesn't fall. And she doesn't stop.

  I follow three rungs below. Ready to catch her if she slips. Ready to push her up if she stalls. Her bag over my shoulder, plus whatever soaked boxes I fished from the water.

  Her arms are shaking by the tenth rung. The adrenaline debt is coming due. But mine I can hardly feel, System enhanced bodies after all were something else.

  When we finally reach the hatch, Sofia shoves it aside, pulls herself through. I'm right behind her.