The United States concealed a project to develop an atomic bomb under the name "Manhattan Engineer District." Popularly known as the Manhattan Project, it carried out the first successful atomic explosion on July 16, 1945, in a deserted area called Jornada del Muerto near Alamagordo, New Mexico.
At 2:45 A.M. local time, the Enola Gay, a B-29 bomber loaded with an atomic bomb, took off from the US air base on Tinian Island in the western Pacific. Six and a half hours later, at 8:15 A.M. Japan time, the bomb was dropped and it exploded a minute later at about 560 to 600 meters over central Hiroshima.
Radioactive debris was deposited by "black rain" that fell heavily for over an hour over a wide area. Thermal Hear is intense thermal heat emitted by the fireball and it caused severe burns and loss of eyesight. Thermal burns of bare skin occurred as far as 3.5 kilometers from ground zero. Most people exposed to thermal rays within 1-kilometer radius of ground zero died. The explosion melted tile and glass and all combustible materials were consumed.
An atomic explosion that caused an enormous shock wave followed instantaneously by a rapid expansion of air called is the blast. This represents roughly half the explosion's released energy.
Maximum wind pressure of the blast was 35 tons per square meter. Maximum wind velocity was 440 meters per second. Wooden houses within 2.3 kilometers of ground zero collapsed. Concrete buildings near ground zero (thus hit by the blast from above) had ceilings crushed and windows and doors blown off. Many people were trapped under fallen structures and burned to death.
People exposure within 500 meters of ground zero was fatal. People exposed at distances of 3 to 5 kilometers later showed symptoms of aftereffects, including radiation-induced cancers.
Symptoms appearing in the first four months were called acute. Besides burns and wounds, they included general malaise, fatigue, headaches, loss of appetite, nausea, vomiting, diarrhea, fever, abnormally low white blood cell count, bloody discharge, anemia, and loss of hair.
Prolonged injuries were associated with aftereffects. The most serious in this category were keloids, cataracts, leukemia and other cancers.
The estimated pre-bomb population was 300,000 to 400,000. Because official documents were burned, the exact population is uncertain. With an uncertain population figure, the death toll could only be estimated. According to data submitted to the United Nations by Hiroshima City in 1976, the death count reached 140,000 by the end of December, 1945.