The four soldiers standing in front of the closed station wore thick plated armor painted in shifting camouflage lines the same shades of beige and steel as the corridor. They carried intimidatingly large assault rifles and scowled at the crowd of a dozen or more pressing in around them.

“I am on the transportation board,” a tall, thin, dark-skinned woman was saying, punctuating each word by tapping her finger on one soldier’s chest plate. “If you don’t let us past, then you’re in trouble. Serious trouble.”

“How long is it going to be down?” a man asked. “I need to get home. How long is it going to be down?”

“Ladies and gentlemen,” the soldier on the left shouted. She had a powerful voice. It cut through the rumble and murmur of the crowd like a teacher speaking to restless schoolchildren. “This settlement is in security lockdown. Until the military action is resolved, there is no movement between levels except by official personnel.”

“Whose side are you on?” someone shouted. “Are you Martians? Whose side are you on?”

“In the meantime,” the soldier went on, ignoring the question, “we are going to ask you all to be patient. As soon as it’s safe to travel, the tube system will be opened. Until that time, we’re going to ask you to remain calm for your own safety.”

Prax didn’t know he was going to speak until he heard his own voice. He sounded whiny.

“My daughter’s in the eighth level. Her school’s down there.”

“Every level is in lockdown, sir,” the soldier said. “She’ll be just fine. You just have to be patient.”

The dark-skinned woman from the transportation board crossed her arms. Prax saw two men abandon the press, walking back down the narrow, dirty hall, talking to each other. In the old tunnels this far up, the air smelled of recyclers—plastic and heat and artificial scents. And now also of fear.

“Ladies and gentlemen,” the soldier shouted. “For your own safety, you need to remain calm and stay where you are until the military situation has been resolved.”

“What exactly is the military situation?” a woman at Prax’s elbow said, her voice making the words a demand.

“It’s rapidly evolving,” the soldier said. Prax thought there was a dangerous buzz in her voice. She was as scared as anyone. Only she had a gun. So this wasn’t going to work. He had to find something else. His one remaining Glycine kenon still in his hand, Prax walked away from the tube station.

He’d been eight years old when his father had transferred from the high-population centers of Europa to help build a research lab on Ganymede. The construction had taken ten years, during which Prax had gone through a rocky adolescence. When his parents had packed up to move the family to a new contract on an asteroid in eccentric orbit near Neptune, Prax had stayed behind. He’d gotten a botany internship thinking that he could use it to grow illicit, untaxed marijuana only to discover that every third botany intern had come in with the same plan. The four years he’d spent trying to find a forgotten closet or an abandoned tunnel that wasn’t already occupied by an illegal hydroponics experiment left him with a good sense of the tunnel architecture.

He walked through the old, narrow hallways of the first-generation construction. Men and women sat along the walls or in the bars and restaurants, their faces blank or angry or frightened. The display screens were set on old entertainment loops of music or theater or abstract art instead of the usual newsfeeds. No hand terminals chimed with incoming messages.

By the central-air ducts, he found what he’d been looking for. The maintenance transport always had a few old electric scooters lying around. No one used them anymore. Because Prax was a senior researcher, his hand terminal would let him through the rusting chain-link fencing. He found one scooter with a sidecar and half a charge still in the batteries. It had been seven years since he’d been on a scooter. He put the Glycine kenon in the sidecar, ran through the diagnostic sequence, and wheeled himself out to the hall.

The first three ramps had soldiers just like the ones he’d seen at the tube station. Prax didn’t bother stopping. At the fourth, a supply tunnel that led from the surface warehouses down toward the reactors, there was nobody. He paused, the scooter silent beneath him. There was a bright acid smell in the air that he couldn’t quite place. Slowly, other details registered. The scorch marks at the wall panel, a smear of something dark along the floor. He heard a distant popping sound that it took three or four long breaths to recognize as gunfire.

Rapidly evolving apparently meant fighting in the tunnels. The image of Mei’s classroom stippled with bullet holes and soaked in children’s blood popped into his mind, as vivid as something he was remembering instead of imagining. The panic he’d felt in the dome came down on him again, but a hundred times worse.

“She’s fine,” he told the plant beside him. “They wouldn’t have a firefight in a day care. There’re kids there.”

The green-black leaves were already starting to wilt. They wouldn’t have a war around children. Or food supplies. Or fragile agricultural domes. His hands were trembling again, but not so badly he couldn’t steer.

The first explosion came just as he was heading down the ramp from seven to level eight along the side of one of the cathedral-huge unfinished caverns where the raw ice of the moon had been left to weep and refreeze, something between a massive green space and a work of art. There was a flash, then a concussion, and the scooter was fishtailing. The wall loomed up fast, and Prax wrenched his leg out of the way before the impact. Above him, he heard voices shouting. Combat troops would be in armor, talking through their radios. At least, he thought they would. The people screaming up there had to be just people. A second explosion gouged the cavern wall, a section of blue-white ice the size of a tractor calving off the roof and falling slowly and inexorably down to the floor, grinding into it. Prax scrambled to keep the scooter upright. His heart felt like it was trying to break out of his rib cage.

On the upper edge of the curving ramp, he saw figures in armor. He didn’t know if they were UN or Mars. One of them turned toward him, lifting a rifle. Prax gunned the scooter, sliding fast down the ramp. The chatter of automatic weapons and the smell of smoke and steam melt followed him.

The school’s doors were closed. He didn’t know if that was ominous or hopeful. He brought the wobbling scooter to a halt, jumped off. His legs felt weak and unsteady. He meant to knock gently on the steel drop door, but his first try split the skin over his knuckle.