One moment he was so close to the fire that his shirt was steaming, the next he was in the tangled dark of the willows forcing his way to the road where he'd left the jeep rather than pass by the corrals, rather than see anyone or have anyone see him. As he stumbled out of the woods, headlights ambushed him, as if the burning cow had risen and stalked him.

The lights swerved. A Buick fish tailed to a stop, its rear end in the mud of the shoulder of the road. Ray Stingo and then Oppy came running to Joe, shouting at the same time.

"You okay?"

"What happened?"

"Doing a little native liaison for you," Joe told Oppy.

"The blood -"

"Why was the cow white?" Joe demanded.

"White -"

"The cow I killed because it was hot."

"Hair can react to low levels of radiation. So it's cow's blood." Oppy stared at Joe. "You should see yourself."

"What are you doing here?" Joe asked Ray.

"We went to the train station at Lamy. Early train from Chicago."

Oppy said, "I told Sergeant Stingo to swing by here on the way back so I could ask you to drive Dr Pillsbury around the high explosive sites today. And remember, you're guarding a party tonight."

"Okay, but I want a weekend pass."

"Joe, we're one month from the test."

"I need a pass."

"Why?"

Joe laboured each word. "To get the blood off."

"I'll do what I can." Oppy looked at the car. "You think you can help us get the car back on the road?"

As the three men walked to the Buick, Joe saw that a rear window was rolled down. Of course, Ray and Oppy had gone to the train to meet a passenger. With the final rush to the test, all sorts of people were coming to the Hill from Oak Ridge, New York, Chicago. In the dimness, Joe recognized her by her cool gray-eyed gaze. Fuchs' partner from the Christmas dance. Joe hadn't seen her since.

"He's all right, Anna," Oppy said. "It's not his blood."

"Whose is it?" she asked.

Joe stopped by the bumper. The rear right wheel had made its own well in the mud.

"Get her out so I can move the car over."

"Dr Weiss?" Ray opened the door for her.

She looked at Joe's shirt and could have been scrutinizing the gore on a beast that walked on all fours. Joe noticed the white azalea in her hair; white azaleas were Oppy's favourite. He could just see Oppy offering it to her as she stepped from the train.

"A real giant would be able to lift me, too."

"Anna," Oppy said, "be reasonable."

"Okay," Joe said. "Stay."

"Joe, if the three of us -" Ray began.

Lift me? Joe gripped the chrome handle of the bumper, rocked the car and tested the suction of the ooze on the tyre. He could lift an elephant and kick its ass down the road. Through the rear window her eyes glittered. On the third push the tyre ripped free of the mud and in the same motion Joe straightened and walked the rear end of the Buick on to the road. When he let the car drop, she laughed, as if nothing he did surprised her, let alone scared her.

"Don't forget Harvey Pillsbury." As Oppy got into the car, he gave Joe a worried glance.

Joe had forgotten Harvey, and the cow. Watching the tail-lights move away, Joe could have sworn he saw the flash of her looking back.

On Two Mile Mesa, south of Los Alamos, bulldozers had cleared pinon, cedar and cactus to make way for test pads and concrete bunkers. There were photo bunkers with spring-forced steel jaws that would snap shut before rocketing debris reached the cameras inside. There were X-ray bunkers, steel-sheathed and coffin-shaped, that resembled ironclad warships sinking into the sand. Plus gauge and meter bunkers. Magazine bunkers. Control bunkers. On the raw plain, the bunkers fought their own war, firing more than ten tons of high explosive a week.

The Hanging Garden was the biggest test site, an entire hilltop shaved level by Jaworski's crew. It looked like an Aztec pyramid forty yards across at the top, but instead of a bloody altar was a steel pad blackened by carbon and fire, and attended not by priests but by a dozen draft-deferred graduate students in shorts and baseball caps. The overall litter of burned cables and broken glass gave a false impression of disorder. There was a pattern. At the outer edges were the periscopes for the flash and rotating prism cameras that would record every microsecond of a blast. Halfway to the pad were deep trenches for pressure gauges. Nearer to the pad, the buried mother cable emerged from the ground to be attached to exposed detonator cables. Almost nudging the pad was an X-ray bunker with the distinctive aluminium nose cone from which the rays would emanate to take their ghostly pictures. On the pad was a waist-high wooden table stamped USED for United States Engineers Detachment and in the middle of the table was a model of a plutonium bomb, a twenty-inch sphere with a steel shell of bright pentagonal plates bolted together at the edges. The team in baseball caps was connecting black cables to the detonator ports in each plate.

Leopold Jaworski wore suit, braces, a military brush of gray hair and moustaches dyed as dark as arrowheads. He had soldiered against Kaiser Wilhelm of Germany, Tsar Nicholas of Russia and Marshal Pilsudski of Poland. In fact, he was the only scientist on the Hill who knew anything about war.

"You see," he explained to Joe, "a uranium device is child's play compared to this. Simply put half your uranium at one end of a barrel, half at the other end, shoot them together with gun cotton and you have your critical mass and chain reaction. But plutonium has to be brought together into a critical mass much faster with high explosive, at 3,000 yards per second. Explosion is not good enough. The explosive in this device crushes and implodes a plutonium core into a critical mass."

"That will take a lot of explosive," Joe said, to sound intelligent.

"The energy released by the nuclear fission of one kilogram of plutonium is equal to 17,000 tons of TNT."

Joe nodded to the model on the pad. "You don't have a plutonium core in this, do you?"

"No." Harvey arrived, puffing; he'd gone back down to the jeep for his clarinet, which he carried around like a riding crop. "Leo wants to blow up the table, not the mesa."

"I used a squash ball for this test," Jaworski said. "I assume the core in the full model will be the size of a croquet ball."

"About," Harvey said.

"About?" Jaworski sounded horrified and delighted at once. "Dr Pillsbury, here you are, head of the schedule committee, and you don't know how large the core will be? Isn't the core your very particular assignment?"

"There'll be enough credit to go around if the gadget fizzles."

"Harvey, if this 'gadget fizzles', no one will ever, ever hear of it. The Manhattan Project will be the American doughnut hole of history."

"What are you testing now?" Harvey asked to change the subject.

"Ah, now? We are testing some new detonators that must fire through a bank of high-voltage condensers in the same one-millionth of a second. We are testing lenses of Baratol explosive to focus the shock wave. And we are testing a flash technique for shadow photography."

"We have thirty days until Trinity. All this information is absolutely necessary?"

Jaworski turned to Joe. "Hitler goes to hell. The devil takes him to different rooms to choose his punishment. In the first room, Goering is nailed to a wheel and rolled through boiling oil. In the second room, Goebbels is being devoured by giant red ants. In the third, Stalin is making love to Greta Garbo. 'That's what I want,' Hitler says, 'Stalin's punishment.' 'Very well,' says the devil, 'but, actually, it's Garbo's punishment.'" Jaworski turned back to Harvey. "It helps to have all the information. Don't worry, I've tested weapons for thirty years. I know the military mind. General Groves wants this bomb. I'm confident he will drop something on Japan."