The three-square-mile region obliterated by the bomb stretched just over a mile east to west and three miles north to south, from the top edge of the Urakami Valley to the bay, close to where Nagano stood. The affected area would have been significantly larger, as in Hiroshima, had not the blast, heat, and radiation been contained by the mountains surrounding the valley to the north, east, and west. The location of the bay, a mile and a half south of the hypocenter, also limited the extent of the bomb's destruction.

In both Hiroshima and Nagasaki, fatalities, injuries, and physical destruction from the bombs' blast, heat, and radiation are described in relation to distance from the atomic explosion, creating an imagined overlay of concentric circles radiating outward from the hypocenter. Within Nagasaki's first concentric circle -- a half kilometer (three-tenths of a mile) in all directions from the blast -- nearly all buildings were demolished, and bodies were disintegrated or burned beyond recognition. Mortality was estimated at over 90 percent.

Yoshida, Do-oh, and Taniguchi were north of the explosion. The well where Yoshida had stood was in the second concentric circle, its outer boundary marking a radius of one kilometer (six-tenths of a mile) from the hypocenter. There the blast pressure tore off heads and limbs and caused eyes and internal organs to explode. The bomb's heat scalded the water in a nearby pond and caused terrible burns on the bodies of children playing by the shore. A woman who had covered her eyes from the flash lowered her hands to find that the skin of her face had melted into her palms. Most trees were downed or shattered. Thousands of people were crushed beneath toppled houses, factories, and schools, and thousands more suffered severe thermal burns. Roof tiles blistered in the heat.

Like everyone else in Nagasaki that day, Yoshida's immediate survival and degree of injury from burns and radiation depended entirely on his exact location, the direction he was facing in relation to the bomb, what he was wearing, and what buildings, walls, trees, or even rocks stood between him and the speeding force of the bomb's titanic power. Yoshida had been facing in the direction of the hypocenter only a half mile away in a rural region of the Urakami Valley, with very little to shield him from the bomb's blast force and heat. "I was hurled backward into a rice paddy, right? At some point I regained consciousness and could feel the coldness of the water. I stood up, and my body was covered in mud." The skin on his arm had peeled off and was hanging down from his fingertips, and he could feel that his chest and legs were burned, but Yoshida did not yet know the extent of his facial burns. "Blood was pouring out of my flesh," he said. Like thousands of others, he went into shock. "I know it sounds strange, but I felt absolutely no pain. I even forgot to cry."

The blast had thrown Yoshida and his friends in different directions, but all six survived, albeit with serious burns and wounds. After some time, they found one another and slowly made their way to a small tributary of the Urakami River, where they rinsed the mud off their bodies and lay down together in the grass, hoping that someone would find them. One of Yoshida's friends handed him a broken piece of mirror, and when Yoshida looked at his reflection, he could not comprehend what he saw. "All I can say is that I didn't recognize my own face."

Hundreds of field-workers and others staggered by, moaning and crying. Some were missing body parts, and others were so badly burned that even though they were naked, Yoshida couldn't tell if they were men or women. He saw one person whose eyeballs hung down his face, the sockets empty. Everyone begged for water, and some died while drinking from the stream. During the war, Yoshida's teachers had incorrectly warned their students that drinking water while injured would cause excessive bleeding and death -- so Yoshida held out all day with no water to ease his extreme dehydration.

A group of wailing mothers coming down from the mountains shook Yoshida and his friends out of their dazed state and awakened them to their own physical pain and terror. "We were only thirteen years old," he recalled, "and when we heard these mothers crying, we started sobbing too, even louder than they were." The boys rose to their feet and followed the women down the slopes toward the city. At the Urakami River, however, Yoshida wavered. Clutched by pervasive heat and choking dust, he saw people on the ground with severed limbs and heads split open, their brains oozing out. Other bodies were completely carbonized -- "turned into charcoal," he remembered. The river, to which people had fled for relief from the heat and intense thirst, had become a mass grave -- because they drank the water, Yoshida thought. Corpses bobbed in the river, now red with blood.

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Yoshida began to feel his face and body swell. He looked down to see leeches from the rice paddy stuck to his bare legs. He and his friends stumbled back to the small stream where they had come from, removed the leeches from their bodies, and placed uncharred leaves over their raw flesh. In an attempt to escape the heat of the sun, the seven boys burrowed into the tall grass against the riverbank. Pain shot through Yoshida's body, and by this time, his face was so swollen that he couldn't see. "Hang in there, okay?" the boys whispered to one another. "Gotta keep going -- do our best to stay alive!"

From "Nagasaki: Life After Nuclear War" by Susan Southard. Reprinted by arrangement with Viking, an imprint of Penguin Random House LLC. Copyright © 2015 by Susan Southard.