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5 August 2015 Last updated at 19:05 BST
She discovers what it was like first hand from one survivor who was just 14-years-old at the time.
We also talk to two young Japanese girls about what Hiroshima means to them.
Watch Newsround's special programme about Hiroshima.
Bun Hashizume's books are published by Koubunken Co., Ltd.
On the 70th anniversary of the atomic bombing of the city of Hiroshima, Leah travels to Japan to find out more about what happened on that day in 1945.