ground zero

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. The target of a projectile, such as a missile or bomb.
• n. The site directly below, directly above, or at the point of detonation of a nuclear weapon.
• n. The center of rapid or intense development or change: "The neighborhood scarcely existed five years ago, but today it is the ground zero from which designer shops and restaurants radiate” ( Robert Clark).
• n. The starting point or most basic level: My client didn't like my preliminary designs, so I returned to ground zero.

from Wiktionary, Creative Commons Attribution/Share-Alike License

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

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

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