Filter life: 43 minutes, 17 seconds.

Mira had no filter of her own.

Rina’s auxiliary circulation was falling again from 37 percent.

The nearest sealed facility to the outer wall was the heat-exchange station to the northwest. Map distance: 680 meters. But the map showed no mud, no white stalks grown waist-high, no holes waiting to swallow the culture tank’s wheels.

“Three.”

Yona handed her the mask.

Mira put it on and breathed in three times. The sting in her throat eased a little. She held the fourth breath and passed the mask back.

“At this rate, neither of us will make it.”

“Then keep wearing it.”

“If you collapse, no one knows the way.”

“The way is on the map.”

Yona pointed to the water spread beneath the outer wall.

“The map says that’s solid ground.”

Black specks moved beneath the white film on the rainwater. Starting where Rina’s biological lines had touched, they formed a thin band toward the ruins. The colony did not follow the flow. It was climbing against it, toward higher ground.

Ian started to crouch. Mira said, “Don’t touch it.”

The boy folded his hands behind his back.

“I wasn’t going to.”

Mira lowered the hand she had not used to stop him.

Daniel and three outer-wall workers laid metal plates from a decommissioned line beneath the culture tank’s wheels. They made a road by moving the plates forward one at a time. Every time they pulled one from behind and set it in front, the tank advanced seventy centimeters.

Sarah checked the control panel.

“Rina, circulation on the right is at 31 percent. At this speed, it’ll take more than fifty minutes to reach the heat-exchange station.”

“Push faster.”

“If it shakes, the tube will kink again.”

“Then shake it less and push faster.”

Daniel exhaled.

“If we could do that, we’d already be doing it.”

Rina looked at her father through the glass.

“Are you angry?”

“I’m scared.”

Daniel lifted a metal plate and moved it forward.

“But you said we should go. So we’re going.”

The first wheel left the drainage channel beneath the outer wall and touched mud. It sank two finger joints deep.

One of the workers swore. His name was Bek Sol. He had maintained City Seven’s outer-wall floodgates for eighteen years, but he had never worked on actual soil.

“This is ground?”

Yona said, “Ground after rain.”

“What is it like normally?”

“Harder. More cracked.”

“Which is better?”

“Both are bad for wheels.”

They pushed the culture tank. The end of the metal plate slid into the mud. The tank did not move.

Rina’s heart rate climbed to 151.

Ian gripped a handle.

“I’ll push too.”

Mira looked at his fingers. The third and fourth fingers of his right hand would not straighten completely.

“Use your left hand.”

“I can use my right.”

“If it hurts, you’ll let go.”

“Anyone could let go.”

Mira closed her mouth. Ian took the handle in both hands. Whenever his right hand weakened, he shifted the position of his left.

“One, two.”

Eight people pushed.

The culture tank broke free of the mud. At the same moment, the metal plate behind it sprang up and struck Bek’s shin. He dropped to one knee.

“The bone?” Sarah asked.

“Not broken.”

His work clothes had torn. Red spots spread from where rainwater touched his skin.

Sarah started to rinse it with water, then stopped. The only clean water they had was 1.4 liters reserved for flushing Rina’s infusion tube.

“We don’t have water to clean it.”

Yona collected rainwater from the edge of the tarp and smelled it.

“Use this to get the mud off first.”

“It may introduce more external microorganisms.”

“They’re already in.”

Bek rolled up his pants. Fine white lines appeared between the red dots on his skin.

“Are these breathing tubes?”

Sarah touched them with a gloved hand.

“They’re attached to the surface, not under the skin.”

“Pull them off.”

“If I pull them off as they are, the skin may come with them.”

“Then what do you do?”

“I don’t know.”

Bek glared at Sarah, then nodded.

“That, I can believe.”

Filter life: 38 minutes, 2 seconds.

Distance to heat-exchange station: 603 meters.

Noah’s voice came over the comm. People coughed behind the static.

“Holding-area oxygen is at 17.9 percent. Estimated air-cell duration is one hour, forty-one minutes.”

Mira asked, “Is city air coming through the gap around the inner door?”

“The pressure has equalized. There is almost no flow.”

“Can you open the door farther?”

“The manual shaft is on the inside. If I enter, I will not be able to return.”

Someone shouted behind Noah.

“Tell them to send us back into the city!”

Another person said, “They went out. Open it to the outside for us too!”

Noah muted the comm for a moment.

“They have split into two groups. One wants to open the inner door. The other wants to reopen the external door.”

“The external door has no power.”

“People are trying to break the manual shaft.”

Mira looked at the closed outer-wall door. The locking shaft was not visible from this side.

“Can you stop them?”

“Is stopping them the correct action?”

Mira started to answer, then stopped.

“How many minutes do I have to decide?”

“Approximately forty before the air deteriorates.”

“Tell them everything. Where each door opens, how many filters there are, the condition of the city-side sector. Then call me again.”

“Understood.”

Mira ended the transmission.

Yona said, “While that happens, we can’t reach the heat-exchange station.”

“We’re going.”

“If they say they’re opening the door, will you go back?”

“We’ll decide then.”

“Can’t we decide in advance this time?”

“If we decide in advance, someone else decides for them again.”

“Thinking ahead isn’t the same as deciding for someone.”

Yona handed Mira the mask again.

“You keep making them the same thing.”

Mira took three breaths.

The group moved away from the outer wall, shifting the metal plates as they went. The rain thinned, but the wind grew stronger. The filter display dropped two minutes at a time. The spore load was higher than expected.

Worker Jin Ara looked back at the outer wall.

“I never knew the city was this small.”

Seen from inside, the outer wall had been the end of the world. From outside, it was a low structure embedded in a gray plain. Its upper half was hidden by rain clouds, and old solar collectors peeled away like scales.

“Do all twelve cities look like that?” Ian asked.

Yona said, “I’ve only seen three. Part of City Three’s wall is gone. We couldn’t get close to City Nine.”

“Why?”

“Things outside took it before people could.”

“What things?”

“I don’t know. The probes that went in never came back.”

“People?”

“We didn’t send any.”

Ian moved his fingers as if counting the cities. Two fingers on his right hand lagged behind.

“What about the other nine?”

“They’re on the map.”

“And the people?”

Yona did not answer.

The heat-exchange station was a black structure five hundred meters ahead. Part of the roof had collapsed, leaving the building canted to one side. White stalks and brown leaves grew together through the gap. Whenever the wind blew, silver powder lifted from the undersides of the leaves.

The portable sensor issued a warning.

NICKEL COMPOUNDS.

SILICA DUST.

UNIDENTIFIED PROTEINACEOUS PARTICLES.

Bek coughed. Even behind the mask, his voice cracked.

“Is the filter defective?”

Sarah checked his mask.

“The seal is normal. It may be material that entered through the skin.”

“My skin is making me cough?”

“An immune response does not occur in only one place.”

The red spots that had begun at Bek’s shin had reached his knee. The white lines were no longer confined to the surface. Several had entered along the edge of the wound.

“Cut them out,” Bek said.

Sarah took out scissors.

“The parts already inside the skin may remain.”

“Then cut deeper.”

“I may damage a nerve or blood vessel.”

“Can you treat it if we get back to the city?”

Sarah looked at the closed outer-wall door.

“Even if we can go back, we don’t have a treatment for this.”

Bek took the scissors from her.

“I’ll do it.”

“If you do it alone—”

“It’s my leg.”

He cut shallowly around the skin where the white lines clung. Blood welled. On the severed piece of flesh, the lines contracted, then turned themselves toward the rainwater.

Bek threw it into the mud.

“Done.”

Sarah poured two hundred milliliters of clean water over the wound.

Rina asked, “My water?”

Sarah’s hands stopped.

“I used some of the flushing water. I should have told you.”

“How much is left?”

“1.2 liters.”

“Tell me first next time.”

“I will.”

Daniel looked at Bek’s wound.

“You used Rina’s water on him?”

“If it becomes infected, it may endanger everyone.”

“It still belonged to someone.”

“I just said I was wrong.”

“Does saying you were wrong bring the water back?”

Rina tapped the glass.

“Dad.”

Daniel stopped.

“Let me speak.”

“All right.”

Rina looked at Sarah.

“Using it on Bek is okay. Not asking me first isn’t.”

“I’ll record it.”

“Don’t record it. Ask me next time.”

Sarah closed the water container.

“I’ll ask next time.”

Filter life: 29 minutes, 46 seconds.

Rina’s auxiliary circulation: 24 percent.

A sharp rupture sounded from the culture tank’s outer shell.

Sarah ran to the control panel.

“Stop.”

EXTERNAL PRESSURE: 88.9 KILOPASCALS.

CULTURE-TANK SHELL PRESSURE: 96.2 KILOPASCALS.

The pressure calibrated inside the city was higher than the pressure outside. The buffer membrane between glass and metal had swollen like a balloon and was pushing against the frame. The right wheel had lifted a finger’s width off the ground.

Daniel asked, “Will it burst?”

“The buffer membrane will tear before the glass. If it tears, it may pull the tubing inside the tank.”

“Release the pressure.”

Sarah opened the shell’s pressure-release port, then closed it.

“If I let outside air in directly, spores will be trapped inside the shell. They won’t touch Rina now, but they may enter when we replace the tubing.”

“And if you don’t?”

“One more impact in transit, and I don’t know what will tear first.”

The swollen membrane distorted Rina’s view of the outside. She narrowed her eyes.

“How much do you need to release?”

“About half the difference between city pressure and outside pressure. I need a microfilter for the release port.”

Yona touched her mask.

“Can you cut up a person’s filter?”

“Half a cartridge would be enough. But that filter’s life and flow rate would fall below half.”

The outer-wall workers looked at one another. Their remaining times were similar, but differed slightly with the amount of spores each had been exposed to.

Luis Dan removed his filter.

“Use mine.”

Jin Ara said, “Why yours?”

“I have the best lung capacity of the three of us.”

“That was measured in the city. This isn’t the city.”

“Then should we draw lots?”

Bek raised his injured leg.

“If I offer mine, you’ll say we attached an infected man’s filter to the tank.”

Sarah said, “The inside of the cartridge is probably uncontaminated. I can’t confirm it.”

Luis asked Rina, “Can we use my filter?”

Rina looked at his face. His mask had been off for less than ten seconds, but his eyes were already bloodshot.

“What about you?”

“Jin and I will share hers.”

“Like Mira?”

“We’ll pass it more often than they do.”

Jin said, “I haven’t said I’ll give it to you.”

Luis looked at her.

“Will you?”

Jin checked her filter time.

“Two breaths. Then give it back.”

Rina said, “Then use it.”

Sarah dismantled the cartridge. The internal filter membrane was a pleated layer the size of her palm. When she cut it in half, Luis’s filter displayed an error.

NONSTANDARD FILTER MEDIA.

SEAL INTEGRITY CANNOT BE DETERMINED.

Sarah put the remaining half back and sealed the edge with medical adhesive.

“The machine won’t tell you it’s safe anymore.”

Luis took the filter and put it on.

“Honest, since it isn’t safe.”

Sarah inserted the severed membrane into the release port. She opened the valve two millimeters.

The air made a thin, keening sound.

Buffer-membrane pressure: 95.

93.

91.4.

The outside of the filter membrane turned gray. Spores clung between the pleats, and metal dust covered them.

Sarah closed the valve.

The buffer membrane returned to its original thickness. The lifted wheel touched the ground.

Rina’s auxiliary circulation remained at 24 percent. Releasing the pressure had not treated her. It had only reduced one risk: that the tank would burst.

Luis’s estimated filter life was now 11 minutes, 3 seconds.

He held out a hand to Jin. She took off her mask and passed it to him.

“Two.”

Luis breathed twice and gave it back.

Mira and Yona, Luis and Jin, repeated the same act at different intervals. Each time they removed a filter, they stopped talking. Outside, even conversation consumed air.

They pushed the tank again. When white stalks entered between the wheels, they cut them on the metal plates. Some of the severed stalks stuck beneath the tank. Sarah tried to remove them, but Rina stopped her.

“Leave them for a minute.”

“Why?”

“It’s getting cooler.”

The temperature of the culture fluid was dropping by 0.4 degrees. The white stalks seemed to be carrying heat away from the glass in place of the external cooling plate, which had stopped for lack of power.

Sarah touched a portable meter to them.

“Moisture inside the stalks is evaporating and drawing off heat. They’re absorbing salts from the glass surface too.”

“Is that good?” Ian asked.

“For now. But if they enter the tank, they may trigger an immune response.”

Rina said, “I’ll make sure they don’t come inside.”

“That doesn’t mean you can control them.”

“I know.”

Sarah marked the tips of the stalks in red.

“I’ll cut them if they grow past here. All right?”

“Okay.”

They moved on with three white stalks attached. The culture tank’s power consumption dropped by 11 percent. Auxiliary circulation held at 24 percent and fell no farther.

Instead, a small white spot appeared on Rina’s left wrist.

Its spacing matched the stalks outside the glass.

Sarah did not take a picture. She showed Rina first.

“May I document it?”

Rina looked at her wrist.

“Only in my records.”

“I won’t upload it to the research network.”

“Show Dad too.”

Daniel came close to the glass. He did not touch it.

“Does it hurt?”

“No. That makes it scarier.”

Daniel nodded.

“For me too.”

Distance to heat-exchange station: 211 meters.

The mud ended. Cracked concrete began. The wheels moved faster, but the powder in the wind grew thicker. Estimated filter life fell to seventeen minutes.

The entrance to the heat-exchange station was blocked by a half-collapsed steel panel. Bek and Jin Ara set a manual cutter against it. The blade cut ten centimeters into the panel, then stopped.

“Cell is dead,” Jin said.

The remaining worker, Luis Dan, examined the hinges beside the door.

“If we pull the pins, we can lay the whole door down.”

Mira touched a hinge. Rust came away on her fingers.

“The upper pin is bearing the load. Pull it first and the door falls toward us.”

“The bottom one first?”

“Then it twists and the top one won’t come out.”

“So what do we do?”

The only equipment capable of bracing it was the culture tank.

Daniel said, “No.”

“I’m not putting the tank under the door.”

Mira stood two metal plates in a triangle.

“We use these as braces and transfer the weight of the door to the heat-exchange tower column with the securing straps.”

Yona struck the column. It sounded hollow.

“This is rotten too.”

“It only has to hold once.”

“I’ve heard that a lot today.”

They looped two straps around the door. Bek took the upper pin, Luis the lower. Jin watched the crack in the column and raised her hand.

“Pull.”

They hauled on the straps.

Mira struck the lower pin with a hammer. The first three blows dislodged only rust. On the fourth, the pin shifted two centimeters.

The column creaked.

“Stop,” Yona said.

“Keep going.”

Mira struck again.

The lower pin came out. The door tilted and pressed down on the metal-plate brace. One of the upper straps snapped.

Bek fell. The bottom of the door crushed his boot.

“Pull your foot out!”

“It’s stuck!”

They lifted the door. The crack in the column lengthened.

Ian tried to seize a strap, but his right hand slipped. He caught it again with his left.

Bek pulled his bare foot free of the boot. His sock touched the mud.

“Let go!”

The door fell.

The entrance opened with a crash. At the same moment, the upper section of the heat-exchange tower column broke. Silver powder poured down like a cloud.

Filter life plummeted.

08:11.

“Inside!”

They pushed the culture tank into the heat-exchange station.

The air inside was worse. Metal dust and spores lay in layers over the old cooling plates. But the walls blocked the wind, and there was an internal maintenance room that could be sealed.

Its door was made for people. Width: 1.1 meters.

The culture tank was 1.32 meters wide.

Daniel looked at the frame.

“It doesn’t fit. Again.”

Rina said, “Can’t we leave it outside?”

Sarah raised the sensor.

“The spore concentration in this space is three times higher than outside.”

Mira checked the bolts in the doorframe. Four were rusted. Two were embedded in the wall.

“An hour to widen it through the wall.”

Filter life: 7 minutes, 22 seconds.

Yona pointed deeper into the station.

“Coolant passage.”

It was a low passage that forced a person to stoop. An old transport rail remained on the floor. It was wider than the culture tank.

“Where does it end?”

“External storage tank.”

“Is it sealed?”

“It was seventeen years ago.”

A regular metallic sound came from the passage.

Thud.

Three seconds later, another thud.

Yona pressed her back to the wall.

“Something is moving.”

A low machine emerged beneath the portable light. Two of its six wheels were missing. It dragged itself along the rail on the four that remained.

A water tank was coupled behind it.

Forge markings and a date remained on its surface.

LAST INSPECTION: 2163.

TRANSPORT PURPOSE: EXTERNAL STORAGE REPLENISHMENT.

DESTINATION: RELAY STATION FIVE.

CARGO: 1,800 LITERS PURIFIED WATER.

The machine did not see them. Mud covered its sensor lens. It struck the wall, reversed, then struck it again at the same angle.

Thud.

Mira looked at the water-tank valve.

Inside was the water Rina needed, water for people to wash with, water to rinse the filters.

Yona studied the machine’s old coupling shaft.

“Stop it, and we get the water.”

“And if we don’t?”

“We can follow it to its destination.”

“What’s at Relay Station Five?”

“I don’t know.”

Filter life: 05:48.

The machine struck the wall again.

This time, the impact knocked some of the mud from its sensor.

A red light came on in the lens.

The carrier turned toward Mira.

Behind it, the same metallic sound repeated in the dark.

It was not alone.

Red sensor lights came on one by one in the depths of the passage, beyond the reach of the portable lamp. Two. Five. Eleven. Different tanks and crates trailed behind each machine.

The lead carrier began moving toward the culture tank instead of the wall.