ground zero

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The target of a projectile, such as a missile or bomb.
• noun The site of a terrorist bombing or other violent act of destruction.
• noun The site directly below, directly above, or at the point of detonation of a nuclear weapon.
• noun The center of rapid or intense development or change.
• noun The starting point or most basic level.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The point on the land or water surface below, above, or at which an atomic or nuclear bomb detonates
• noun by extension The location of any disaster

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the target of a projectile (as a bomb or missile)
• noun the site of the World Trade Center before it was destroyed
• noun the point of detonation (or above or below) of a nuclear weapon