A test explosion from April 1954 is shown in this photo from the U.S. Defense Department. On Aug. 6, 1945, the U.S. dropped the first atomic bomb on Hiroshima, Japan, hastening the end of World War II and ushering in the nuclear age. Reuters

Seventy years ago this week, as World War II was drawing to a close, the United States dropped two atomic bombs in Japan. The first target was Hiroshima on Aug. 6, 1945, and the second was Nagasaki on Aug. 9. The explosions wrecked the cities but resulted in an Allied victory.

The atomic bombs were a long time coming. American scientists started work on atomic weapons in 1940, joining together for the secret Manhattan Project. They eventually sent their products to New Mexico, where J. Robert Oppenheimer and his team assembled a workable bomb from uranium and plutonium.

In a news release issued Aug. 6, then-President Harry Truman wrote that the bomb had destroyed Hiroshima. "It is a harnessing of the basic power of the universe. The force from which the sun draws its power has been loosed against those who brought war to the Far East," he said, adding, "If they do not now accept our terms they may expect a rain of ruin from the air, the like of which has never been seen on this earth."

Here are facts about the bombs, collected from BBC, the Brookings Institution and the History Channel:

  • The Manhattan Project cost $2 billion.
  • Truman ordered the bomb dropping after Japan rejected the Potsdam Declaration demanding the nation's surrender.
  • Kyoto was initially considered for the second bomb, but -- as legend has it -- Secretary of War Henry Stimson asked for it to be removed because he'd been there on his honeymoon.
  • The plane that carried the bomb to Hiroshima was named "Enola Gay."
  • On board "Enola Gay" were cyanide pills for the passengers to take if the mission failed.
  • The bombs detonated about 1,900 feet above the ground.
  • About 70,000 people were killed instantly, though thousands of others suffered long-term effects from radiation. Recent estimates have put the death toll at 192,000.
  • More than 60,000 buildings were decimated.
  • The Aug. 6 bomb was named “Little Boy,” and it was uranium-based. The Aug. 9 bomb was named “Fat Man,” and it was plutonium-based.
  • “Little Boy” was about three meters long and weighed more than four metric tons. “Fat Man” was even bigger, at about 3.5 meters long and 4.5 tons.
  • “Little Boy” annihilated about five square miles of Hiroshima, whereas “Fat Man” destroyed only 2.6.
  • Only one man officially survived both blasts -- a man named Tsutomi Yamaguchi. He escaped Hiroshima after the first bomb and went to his hometown, Nagasaki, which was ruined three days later. He died in 2010.
  • Japan agreed to the terms of surrender five days after the second bomb was dropped.
  • The formal agreement was signed Sept. 2.
  • The U.S. has built 4,680 nuclear bombers since then.