The atomic bomb, which was first dropped over a populated area days earlier at Hiroshima, carried the explosive power of 21,000 tons of TNT. Codenamed "Fat Man," the plutonium-based bomb flattened three city miles and killed an estimated 70,000 people after being dropped by a U.S. B-29 bomber on August 9, 1945.

President Harry Truman remarked, "It seems to be the most terrible thing ever discovered, but it can be made the most useful..."

Photo: A column of smoke rises 60,000 feet into the air over Nagasaki, marking the impact of the second atomic bombing of Japan during World War II.