There would be no words to adequately describe that happened within the explosion. No one near ground zero would survive to tell their story. But what is known is that the force of the blast--equaling the detonation of 13,000 tons of TNT--and sheer heat killed at least 80,000 civilians instantly (Atomic bombings of Hiroshima and Nagasaki).
As for the witnesses on the ground far enough away from the explosion to have survived, these are their testimonies.
||VIDEO: B-29s drop an atomic bomb on Japan (World War II Database)
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Marx, Joseph L. Seven Hours to Zero. New York: G.P. Putnam’s Sons, 1967.
"Atomic bombings of Hiroshima and Nagasaki." Atomic bombings of Hiroshima and Nagasaki - Wikipedia, the free encyclopedia. 11 Jul. 2005. Wikipedia. 11 Jul. 2005 <http://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasaki>.