On Aug. 6, 1945, Maj. Gen. Leslie Groves sent a top secret cable to his superiors in Washington, D.C.

In the now declassified cable, Groves, who was in charge of the atomic bombing of Hiroshima, described what had happened.

"First there was a ball of fire, changing in a few seconds to purple clouds and flames boiling and swirling upward," he wrote. "Entire city except outmost ends of dock areas was covered with a dark grey dust layer which joined the cloud column."