What does the term 'ground zero' mean? Where does it come from?

Ground zero is the point closest to the centre of an explosion. It was first used by the Manhatten Project when planning the nuclear bombing raids on Hiroshima and Nagasaki. The Oxford English Dictionary cites a news report from Hiroshima in 1946 as 'that part of the ground situated immediately under an exploding bomb, especially an atomic one.".

In recent years 'Ground Zero' has become synonymous in the public mind with 9/11 and the attack on the Twin Towers. The term was used by news reporters in the immediate aftermath of the explosion and has stayed ever since. Ground Zero has come to symbolise the heart of the conflict between the west and Al Queda and its Islamist affiliates. 
The Looming Tower: Al-Qaeda and the Road to 9/11