In this Monday, Aug. 6, 1945 picture made available by the U.S. Army via the Hiroshima Peace Memorial Museum, a mushroom cloud billows into the sky about one hour after an atomic bomb was detonated above Hiroshima, Japan (Photo: AP)
This Sept. 8, 1945 picture shows an allied correspondent standing in the rubble in front of the shell of a building that once was a movie theater in Hiroshima, Japan, a month after the first atomic bomb ever used in warfare was dropped by the U.S. on Monday, Aug. 6, 1945 (Photo: AP)
File Photo: A pall of smoke lingers over this scene of destruction in Hiroshima, Japan, on August 7, 1945 (Photo: AP)
File Photo: Photo taken from the top of the Red Cross Hospital in Hiroshima looking northwest shows the destruction, 1945 (Photo: US Air Force)
An aerial view of ground zero and the now-famous Atom Bomb Dome in Hiroshima, weeks after the bombing of August 6, 1945. (Photo: U.S. National Archives)
File Photo: An aerial overview of Hiroshima in autumn of 1945. The hypocenter and Atom Bomb Dome are visible at top center. (Photo: U.S. National Archives)
File Photo: The Hiroshima Fire Department lost its only ladder truck when its West Side main fire station was destroyed by the blast and fire of the atomic bomb, 1,200 m (4,000 ft) from ground zero (Photo: U.S. National Archives)
Shortly after 8:15 am, August 5, 1945, looking back at the growing "mushroom" cloud above Hiroshima. When a portion of the uranium in the bomb underwent fission, and was transformed instantly into an energy of about 15 kilotons of TNT (about 6.3 × 1013 joules), heating a massive fireball to a temperature of 3,980 C (7,200 F). The superheated air and smoke rapidly rose through the atmosphere like a giant bubble, dragging a column of smoke up with it. By the time this photo was made, smoke had billowed 20,000 feet above Hiroshima while smoke from the burst of the first atomic bomb had spread over 10,000 feet on the target at the base of the column (Photo: U.S. National Archives)
"Little Boy" unit rests on a trailer cradle in a pit below the open bomb bay doors of the B-29 Superfortress bomber "Enola Gay" on the 509th Composite Group base at Tinian Island in the Marianas Islands in 1945. Little Boy was 3 m (10 ft) long, and weighed 4,000 kg (8,900 lb), but only carried contained 64 kg (141 lbs) of uranium which would be used to create a nuclear chain reaction, and resulting explosion (Photo: U.S. National Archives)
Shortly after 8:15 am, August 5, 1945, looking down on the rising smoke from the atomic explosion above the city of Hiroshima from one of two U.S. Air Force bombers from the 509th Composite Group. By the time this photo was taken, the flash of light and intense heat from a fireball 370 m (1,200 ft) diameter had already taken place, and an intense shockwave radiating out faster than the speed of sound was dissipating, having done most of its damage to ground structures and people in a circle 3.2 km (2 mi) in diameter. (Photo: U.S. National Archives)
People pray for the victims of the 1945 atomic bombing, in the Peace Memorial Park in Hiroshima, in this photo taken by Kyodo August 6, 2014, on the 69th anniversary of the world's first atomic bombing on the city (Photo: Reuters)
A woman prays for the victims of the 1945 atomic bombing, in the Peace Memorial Park in Hiroshima, in this photo taken by Kyodo August 6, 2014, on the 69th anniversary of the world's first atomic bombing on the city (Photo: Reuters)
People wait in queue to offer prayers for the victims of the 1945 atomic bombing, in the rain at the Peace Memorial Park in Hiroshima, in this photo taken by Kyodo August 6, 2014, on the 69th anniversary of the world's first atomic bombing (Photo: Reuters)
Children prepare to offer flowers to the atomic bomb victims during a ceremony at the Hiroshima Peace Memorial Park in Hiroshima, western Japan, Wednesday, Aug. 6, 2014. Japan marked the 69th anniversary Wednesday of the atomic bombing of Hiroshima (Photo: AP)