Ground zero is the exact location on the ground where any explosion occurs.
The term has often been associated with nuclear explosions, but is also used in relation to earthquakes, epidemics and other disasters to mark the point of the most severe damage or destruction. Damage gradually decreases with distance from this point.
The term was military slang.
Many journalists applied the term to describe the former site of the World Trade Center of New York City, which was destroyed in the September 11, 2001 attacks.Rescue workers preferred the phrase "The Pile", referring to the pile of rubble that was left after the buildings collapsed.
http://en.wikipedia.org/wiki/Ground_zero