She’d thought she’d grown accustomed to the blasts of chilly air, but as she crossed the last few strides each wave of pressure felt like a physical assault. She switched on her coherer at the lowest possible brightness and squinted into the machinery where the tunnel began.

In the chamber below her, the pressure of the newly formed gas was increasing, forcing the piston up along the outlet shaft. She could see the great polished stone cylinder rising now, the side of it completely blocking the tunnel, its motion only visible from flaws and scratches rushing by.

Then it cleared the mouth of the tunnel, and the cold air from the chamber came rushing out. Agata felt every hardened patch of wizened skin on her body forced into the flesh beneath, with the chill only sharpening the sensation.

As the pressure driving it plummeted, the piston stopped ascending and came hurtling down. Agata had known the rhythm of the full cycle from the first day she’d crawled into the tunnel, but what mattered now was the exact time the outlet shaft was exposed. She crouched before the piston, utterly attentive, letting the process imprint itself on her, binding every visual and tactile cue into a single act of perception.

She closed her eyes, waited for the moment, opened them: and there it was, the bottom of the piston rising up. She couldn’t lose it now, she couldn’t get it wrong.

Agata drew the canister of disruptor out of her tool belt, and unscrewed the lid to the point where one more quarter-turn would free it. A single knock would spill the contents, but mere passage through the air would not; she’d tested that a dozen times.

She closed her eyes, waited, then opened them. She hadn’t lost the rhythm. Every muscle in her body knew what to do, and when.

The piston plunged, the pressure rose. Agata waited. The piston ascended.

As the piston rose above the mouth of the tunnel, she threw the canister. It entered the shaft and disappeared. She heard no sound from the impact, but she hadn’t expected one; the bottom of the chamber would be far below, and the canister might yet be spinning in mid-air, still caught in the updraught.

The piston crashed down again, closing the mouth of the tunnel. Agata closed her eyes, overcome with relief. She had four more days; she could prepare and deliver at least four more doses. All her numbers were approximations and guesswork, but unless she had wildly miscalculated, that total would at least give her a chance.

The air burst out from the chamber once more; Agata opened her eyes to welcome it. A trace of fine grit stung her eyes, then something clattered loudly across the tunnel floor.