On 6 August 1945, at 8.15am Japanese time, an American B-29 bomber plane, called 'Enola Gay', dropped an atomic bomb on the Japanese city of Hiroshima.
The devastation was beyond anything seen before. The city was immediately flattened.
80,000 people were killed as a direct result of the blast, and another 35,000 were injured.
Leah visited the Hiroshima Peace Memorial Park in Japan, which is a place dedicated to all those affected by the nuclear bombing of the city during World War Two.