The group clambered into the road surface with relief. They started to hike down the road, along the valley toward the lower ground, their mood staying high despite their fatigue.

But the road was in bad shape, Snowy saw. It was overgrown. There was still some asphalt — he could see it as black fragments in the green — but it had aged, becoming cracked and brittle. Plants and fungi had long since broken through the surface, and in fact as he walked he sometimes had to push through thickets of birch and aspen seedlings. It was less like walking along a road than over a sparsely vegetated ridge.

Sidewise was walking alongside him again. “So what do you think? Where are we?”

They had all been trained up in the basic geographical features of Europe and North America. “The valley isn’t glaciated,” Snowy said reluctantly. “So if we’re in Europe, we aren’t too far north. Southern England. France maybe.”

“But it’s been a long time since anybody maintained this road. And look down there.” Sidewise pointed to a line etched in the side of the far valley wall, just bare rock.

“So what?”

“See how level it is? I think this valley was flooded once. Dammed. At the water’s surface you get a lot of erosion — you get horizontal cuts like that — because when the flow is managed, the water levels fluctuate fast.”

“So where’s the dam?”

“We’ll come to it,” said Sidewise grimly.

After another hour of walking, they did.

They turned around a breast of the valley, and there it was. A branch of this roadway actually led down to the dam, and must have run over it to the valley’s far side.

But the dam was gone. Snowy could make out the piers that still clung to the shore, heavily eroded and overgrown with greenery. Of the central section, the great curving wall and gates and machinery that had once tamed the river, there was nothing left but a hummocky arced line on the valley floor, a kind of weir that barely perturbed the river as it ran over it.

Moon said, “Maybe somebody blew it up.”

Sidewise shook his head. “Nothing is impervious. There are always cracks and weaknesses, places the water can get into. And if you don’t do anything about it, the leaks get worse, until…” He fell silent. “All you need is time,” he finished lamely.

“Fucking hell,” growled Bonner. “Fucking buggering hell.”

It seemed to Snowy that the unavoidable truth was starting to sink into them all. Even Sidewise didn’t need to say any more to make it so.

Ahmed strode ahead a few paces, and peered further down the valley. He was a pilot; like them all, he had good eyes. He pointed. “I think there’s a town down there.”

Maybe, thought Snowy. It was just a splash of greenish gray. He could see no movement, no car windshields or windows glinting, no smoke rising, no lights. But they had nowhere else to go.

Before they left the higher ground Ahmed fired off a couple of the search-and-rescue flares he had retrieved from the shelter. There was no reply.

They followed Ahmed as he made bold, defiant strides along the grassed-over roadway, down the valley toward the town. The light began to fade. Not a single light came on in the town they approached; it was a well of darkness and silence.

In some places the river’s banks had reverted to marsh, with low, green-clad hummocks marking what might once have been buildings. Elsewhere the banks were lined with elder and graceful willows — old-looking willows, Snowy thought reluctantly — and the floodplain beyond was covered by a forest of poplar and ash. Beyond, he could see arms of the oak forest spreading over the low hills.

Long before they reached the center of the town they had to abandon the grown-over road, as it slipped under the surface of the broadening river. Further out into the river, Snowy could make out shapes, lines, under the shallow water.

“If you build around a river,” said Moon slowly, “you reclaim the land to either side. Right? But when you abandon the town, the water table is going to rise because you’re no longer pumping out groundwater for industrial use, and you’ll get flooded out.”

Nobody commented. They walked on, skirting the river and its marshy fringe.

At last they came to the town itself. There was a layout of streets here, you could see that, a roughly rectangular grid laid out over shallow slopes. But the roads were as ruined as the one they had followed here. The buildings themselves were just patterns of mounds and hummocks draped with green, most of them no more than waist height. The whole place looked like an overgrown graveyard. Snowy thought they could have passed by any of these heaps of green-clad rubble in the forest and thought it just another extrusion of rock, the product of nature’s mindless churning. Even the vegetation was much the same as in the open land beyond the town. It was only the patterns that told you that hands had built this place, that minds had planned it.

Here and there, though, more enduring fragments poked out of the drowning green. There was one looming, circular hill, as green clad as the rest. Snowy wondered if this might be a keep, the base of one of the Normans’ great castles, erected to enforce their occupation of England in the eleventh century. If so, it had lasted where much else had failed. They came across a row of columns, worn to stubs, that looked as if they had been clad in marble. They might have been the grandiose frontage of a bank or town hall.

And here was a statue, fallen on its back. Its face, pocked by lichen and eroded beyond recognition, peered up at the sky from an ocean of green. But the statue bore traces of charring, Snowy saw. He searched for a date, but couldn’t find one.

When he dug into the greenery that blanketed other anonymous mounds, he found more traces of fire, of soot and scorching. This place had burned, then, before it had been broken up. He was walking on tragedy, on overgrown horror. He wondered how deep he would have to dig before he found bones.

They came to a comparatively open space. This must have been a central square, maybe a marketplace. Ahmed called a halt. They dropped their packs, drank their water, and peered around. In the lengthening shadows of evening the ruined town was an eerie place, Snowy thought, neither quite natural nor human, neither one thing nor the other.

A little ratty creature scuttled from under Snowy’s feet, crisply pattering over the broken asphalt surface and disappearing into the richer green away from the square. It looked like a vole. And, following its tracks, Snowy made out the upright, wary form of a hare. With bewildering speed it turned and scuttled away.

“Voles and hares,” he muttered to Sidewise. “I thought we’d see cats and dogs.”

Sidewise shrugged, sweat and grime coating his face. “People have gone, right? Civilization has fallen, blah, blah, blah. Cats and dogs were pampered, domesticated, all the genetic variation bred out of them. They wouldn’t have lasted long without us.”

“I’d have thought cats would survive. Even little kittens used to go hunting.”

“Wild cats were perfect killing machines. But the domestic variety had smaller teeth, jaws, brains than their wild ancestors, because old ladies liked them better that way.” Sidewise winked. “I always thought cats were faking it. They weren’t so tough. Just a pain in the arse.”

“Where are the cars?” Moon asked. “I mean, I see the buildings, what’s left of them. What about the cars?”

“If you dig in the greenery you might find a few patches of rust, or bits of plastic.” Sidewise glared at Ahmed. “What, are you going to chew me out for lowering morale again? I’m only pointing out the bleeding obvious.”

“But we don’t have to deal with that right now,” Ahmed said, with an evenness Snowy admired. “What we need to do is obvious too.”