In August 1945, these two atomic bombs devastated the Japanese cities of Hiroshima and Nagasaki.
Kohei Oiwa says he witnessed people dying on the streets after the bomb was dropped when he was just 13.
"It was a really hot day and my mother knew we didn't really study at school. So, when I told her my stomach hurt, she said 'you can take a day off today'. These words eventually became a life-or-death decision for me," he says.
As the horror of that day recedes in history, Mayu Yoshida talks to survivors who are keeping their memories alive, and their children and grandchildren who are helping them.