The father looked up. "Good weather. Bombers can't fly and it's easy tracking in the snow."

Joe didn't see any signs. He was a fair tracker, but he was no Apache.

"Better you get the horses than no one."

Finally, the Navajo shivered and lowered his rifle. The four men smoked, contemplating the quiet between the low sky and snow-covered ground. Then the father and son killed their butts and nodded to Joe. The Navajo followed. The three men, the Navajo on the outside, moved off to the north, making a wide arc round the car. Wouldn't that have been an interesting end to the atom bomb, though, Groves and Oppy gunned down in the snow in return for sheep?

Joe opened the general's door. "I don't think they recognized you, sir. I told them they were trespassing on the Alamogordo Bombing and Gunnery Range and would have to leave."

"Seemed a little touch and go," Oppy said.

The Apaches and their friend were already moving out of sight, not so much getting smaller as disappearing between points of snow. The real horizon could be 500 yards away, 1,000, a mile. Oppy emerged from the car, lit a cigarette and lit Fuchs' cigarette, too, with a flourish of relief, as he got out. Groves stepped on to the snow and tilted his head back to perform a professional sweep of the four directions.

Oppy spread the map over the hood. "This is where we are. Latitude 33 40' 31", longitude 106 28' 29"."

"So, where is that?" Groves asked.

"East are the Oscura Mountains." Joe pointed. "South, Mockingbird Gap; west, three volcanoes the locals call Trinity; north, Stallion Gate." Each way was a wall of white.

Where Joe had pinned the map down with his finger, Fuchs 'made an X with a soft pencil and drew a perfect, freehand circle around the X.

"If this is Ground Zero, the point of detonation, we will desire a distance of ten kilometres to the first control shelters."

Groves set a surveying transit in the snow, planting the three legs firmly. The air bubble sat in the middle of the transit level. Flat ground. Confidence was the general's face; he sniffed the air with anticipation. The errant party from Alamogordo was forgotten.

"Just the way we chose Los Alamos," he called to Oppy, "the top men on the spot."

While Groves sighted through the transit telescope, Joe paced off fifty yards with a tape, flags, stakes. Oppy and Fuchs paced off in another direction.

When Groves waved, Oppy set a red flag at Joe's feet.

"Captain Augustino tells me there's a spy on the Hill," Joe said.

"Did he say who?" Oppy looked up with the eyes of an innocent.

"No," Joe lied.

"No names at all?"

"Let's say the person was just a security risk."

"He'd have to be pulled off the project."

"His reputation?"

"Ruined. No names?"

"Let's say I wanted off the Hill. Say I wanted combat."

"That's an Army matter, Joe. The Hill is an Army base, after all. You'd have to go to the head of military administration."

"That's Augustino again."

"The captain is a powerful man in his own little realm."

"Which is the Hill."

"He really didn't give you any names?"

"I suppose he'd tell you if he had a name in mind."

"True." Oppy was relieved. He gave Joe a conspiratorial grin. "Remember, the captain is an intelligence officer. It's his duty to be paranoid."

With the next set of flags, Oppy and Fuchs swapped place's.

"It must be interesting to be an Indian." Fuchs followed Joe's measured steps. "To be free of civilization, to live simply as men and women with nature."

"You mean, go naked?"

"No, I mean defy all bourgeois standards of behaviour. You understand what I mean by bourgeois?"

Joe watched Oppy slowly pacing through the snow. A frail figure, his coat whipping around him. He spread his arms, turning, holding flags, and seemed, in his ungainly way, to be dancing in the snow.

They created a model of the test site to come, red flags for directions, lettered stakes to indicate relative distances to control shelters, base camp, observation posts, evacuation roads, populated areas. By the time they gathered by the transit, the model's Ground Zero, snow had almost stopped. Groves' manner was brisk and expansive, an engineer breaking ground. Waving his hand, he described the test tower, miles of wire, roads and trucks he saw in his mind. Oppy had brought a bottle of cognac, and even Groves, who usually drank nothing more than the smallest glass of sherry, accepted a ceremonial sip. Alone in the car, Joe radioed the convoy that was supposed to have met them hours before. He opened his own flask. Vodka. This was not sophistication. Wartime distillers made vodka from potatoes, corn, molasses, grain. From ethane, methane and petrochemicals. From horse sweat and purified piss. Santa Fe liquor stores wouldn't sell a bottle of anything unless you bought a bottle of vodka. Another subversive communist connection.

He drank from his flask while he fished in the static.

"… difficulty… lost a drive wheel… soon, over."

Joe read and repeated his map co-ordinates to the static and signed off. The general would miss his flight; he'd have to see Roosevelt another day.

Suddenly it was colder and darker. Clouds flowed by on either side, and directly above was a stream of evening stars. When Joe returned, he made a fire from cow pats he dug out of the snow. The other three, exhilarated from mapping the test site, were still sharing the cognac. It occurred to Joe that these minutes of waiting for the party from the base probably were the first moments of relaxation, of complete and powerless rest, that either Oppy or Groves had had in years.

"You have to wonder whether the Chinese alchemists who discovered gunpowder," Oppy said, "when they were on the verge of discovering gunpowder, were fortunate enough to have a night as quiet and beautiful as this. Perhaps the Emperor of China had horsemen searching for them, as jeeps are searching for us. Perhaps we'll meet them."

"What do you mean?" Groves asked.

"Einstein says time bends around the universe in a curving line. On that line you can go backwards or forwards. We'll never find this same Stallion Gate here again, but we can always find it on some cusp of time. If we could do that, we could meet those Chinese horsemen, too."

"I'll tell you about going back in the past," Groves snorted and filled his hand with caramels from his pocket. "The bitterest day of my life was when I was ordered to rescue this project. I had just been offered my first combat command the week before. A soldier wants to see combat. My father was an Army chaplain, and even he saw combat. There I was, Army born and bred, ordered to spend history's greatest war at home overseeing a bunch of scientific prima donnas who, as far as I could tell, had sold the President a bill of goods." He popped a caramel into his mouth and ruminated. "Well, I don't run phoney projects that don't show results. A lot of scientists and so-called geniuses tried to sell me a bill of goods on how to make this atomic bomb. The greatest American physicist is E. O. Lawrence. I like Lawrence. He put the cyclotron on the map and he won the Nobel Prize, but he's hardly produced a speck of uranium. Nevertheless, I will make this project a success. It's largely a matter of plumbing, albeit complicated." Oppy's eyes glittered with amusement. Groves wiped his fingers in the snow. "In fact, I have never been more positive of success than I am at this very moment, at this very place."

"This will be your monument," Oppy said.

"Monument?" Groves sighed. "After I built the Pentagon, I calculated that in my career I had moved enough earth and laid enough cement to build the pyramids of Cheops two hundred times over."

"This is a different kind of pyramid," Oppy suggested. "This has different blocks, some steel, some gold, some of water, some so radioactive you can't touch them or even come close to them, and the pyramid must be built according to a blueprint no one has ever seen."