Snowy nodded. “We have to find shelter.”

Bonner clambered up onto a low mound that might once have been a wall. Now he pointed, roughly west. “That way. I can see walls. I mean, standing walls. Something that isn’t all covered in shit.”

With an unreasonable spark of hope, Snowy got to his feet. It was a church, he saw. A medieval church. He could make out the tall, narrow windows, the high doorway. But the doors and roof had long gone, leaving the building open to the sky. He felt disappointment — and yet a stab of admiration.

Sidewise seemed to share his thought. “If you’re going to build, build out of stone.”

“Where do you think we are? England, France?”

Sidewise shrugged. “What do I know about churches?”

Ahmed picked up his pack. “All right. There’s no roof, so we’ll have to make lean-tos. Bonner, Snowy, come with me and we’ll fetch some branches. And we’ll need a fire. Moon, Sidewise, you attend to that.” He looked around at their faces, which were shining like coins in the gathering dark. This would be the first time they had been out of each other’s sight since they had woken up, and even Snowy felt a pang of uncertainty. “Don’t go too far,” Ahmed said gently. “We’re alone here. There doesn’t seem to be anybody to help us. But we’ll be fine so long as we’re careful. If anything goes wrong — anything — shout or use your pistol, and the rest of us will come running. All right?”

They nodded and murmured. Then they moved off into the gathering dark, purposefully pursuing their allotted tasks.

The interior of the church was just another patch of greenery. There was a mound at one end that might once have been an altar, but there was no sign of pews or crucifixes, prayer books or candles. The roof was gaping open to the sky, not a trace left of the wooden construction that must once have spanned these slender, sturdy walls.

Under their lean-tos, on pallets of brush, with leaves for blankets, it wasn’t going to be such an uncomfortable night. They had all had plenty of survival training; this wasn’t so bad compared to that.

They stuck to their survival packs, munching on dried bananas and beef jerky. They didn’t eat any of the fruit from the forest. It was a little superstitious, Snowy thought, as if they wanted to cling to what was left of the past as long as possible, before committing themselves to this peculiar new present. But it was OK to take it slow. Ahmed was showing a good grasp of psychology in allowing that. It certainly wouldn’t make any difference in the long run.

They were all pretty exhausted after a walk of many klicks on their first day out of the Pit. Snowy wondered how they would have got on if they’d really had to fight; maybe this strategy wouldn’t have worked as well as the planners had imagined. And they all had trouble with their feet, with blisters and aches. It was the lack of socks that was the problem. Snowy worried about using up their limited supplies of ointments too fast. They would have to do something about that tomorrow.

But it was comforting to shelter in this relic of human construction, as if they were still cradled by the civilization they had come from. Still, they would keep their fire burning all night.

Snowy was relieved to find he was too tired to think too hard. Still, he woke.

He rolled on his back, restless. The air was hot — too damn hot for an English spring; maybe the climate had changed, global warming gone crazy or somesuch. The sky framed by the open roof was littered with stars, obscured here and there by cloud. There was a crescent moon, too narrow to banish the stars, as far as he could see unchanged from the patient face that had watched over his boyhood. He had learned a little astronomy, during training exercises in the desert, for navigation purposes. He picked out constellations. There was Cassiopeia — but the familiar W shape was extended by a sixth star. A hot young star, maybe, born since he had gone into the Pit. What a strange thought.

“I can’t see Mars,” Sidewise whispered, from out of the dark.

That startled Snowy; he hadn’t known Sidewise was awake. “What?”

Sidewise pointed to the sky, his arm a silhouette. “Venus. Jupiter. Saturn, I think. Where’s Mars?”

“Maybe it set already.”

“Maybe. Or maybe something happened to it.”

“This is bad shit, isn’t it, Side?”

Sidewise didn’t reply.

“Once I saw some Roman ruins,” Snowy whispered. “Hadrian’s Wall. It was like this. All grown over, even the mortar rotted away.”

“This was a different scale,” Sidewise murmured. “Even from Rome. We had a global civilization, a crowded world. Everything was linked up.”

“What do you think happened?”

“I don’t know. That fucking volcano, maybe. Famine. Disease. Refugees everywhere. War in the end, I guess. I’m glad I didn’t live through it.”

“Shut up, you two,” Ahmed murmured.

Snowy sat up. He peered out through a glassless window frame in the wall of the church. He could see nothing. The land was just a blanket of dark, no glimmer of lights, no glow of streetlights on the horizon. Maybe everywhere was dark like this. Maybe their fire was the only light in England — on the whole damn planet. It was a stupendous, unbelievable, unacceptable thought. Maybe Sidewise could grasp it properly, but Snowy sure couldn’t.

Some kind of animal howled, out in the night.

He threw a little more wood on the fire, and buried himself deeper in his mound of greenery.

Sidewise had been right. Mars was missing.

The replicators, Ian Maughan’s robot probes, had survived. The program had been designed as a precursor to human colonization of the planet. The replicating robots would have been instructed to build homes for human astronauts, to make them cars and computers, to assemble air and water, even grow food for them.

But the humans never came. Even their commands ceased to be received.

That wasn’t troubling, for the replicating robots. Why should it be? Until they were told otherwise, their only purpose was to replicate. Nothing else mattered, not even the strange silence from the blue world in the sky.

And replicate they did.

Many modifications were tried, incorporated, abandoned. It did not take long for a radically better design to converge.

The replicators began to incorporate the factory components within their bodies. The new kind looked like tractors, pilotless, trundling over the impassive red dust. Each weighed about a ton. It took each one a year to make a copy of itself — a much shorter reproduction time than before, because they could go where the resources were.

After a year, one of the new replicator types would become two. Which after another year had made two more copies, a total of four. And in another year there were eight. And so on.

The growth was exponential. The outcome was predictable.

Within a century the factory-robots were everywhere on Mars, from pole to equator, from the peak of Mons Olympus to the depths of the Hellas crater. Some of them came into conflict over resources: There were slow, logical, mechanical wars. Others began to dig, to exploit the deeper materials of Mars. If you mined, there was still plenty of resources to go around — for a while, anyhow.

The mines got deeper and deeper. In places the crust collapsed. But still they kept digging. Mars was a cold, hard world, rocky for much of its interior. That helped the mining. But as they dug deeper and encountered new conditions, the replicators had to learn quickly, adapt. They were capable of that, of course.

Still, the penetration of the mantle presented certain technical challenges. The dismantling of the core was tricky too.

Mars weighed one hundred billion billion times as much as any one of the tractor-replicators. But that number was small in the face of the doubling-every-generation rule. Because of the continuing conflicts, the pace of growth was a little slower than optimal. Even so, in just a few hundred generations, Mars had gone, all but a trace of its substance converted to the glistening bulks of replicators.