The world had only three days to get used to the idea of nuclear weapons... when the second bomb dropped.
It's been 70 years nearly to the day since an American atom bomb exploded over Nagasaki, Japan.
And plenty of survivors are still around to talk about that day.
Susan Southard talked to five of them about that day and the present day, for her book Nagasaki.
The bomb clearly retains the power to shock, even seven decades later. Susan visits the Exchange to explain.
