Place in the Pentagon
where people came to eat lunch
or smoke a cigarette
. Employees in the Pentagon called it ground zero
because it was a quiet center in the middle of all sorts of military
activity. It is said that in a mid-air explosion
the ground directly below the blast remains untouched. This place in a (nuclear) blast-site is also called 'ground zero'.
The name was used until september 11th 2001. Ground zero was then used to refer to the remains of the World Trade Center (Twin towers) and to the place where a plane had hit the Pentagon itself.