ground zero

Short definition from Wiktionary

Noun

1. 1
   Originally, the point on the land or water surface below which a nuclear bomb detonates in the air; now also the point on such a surface at or above the detonation.
2. 2
   The location of any disaster or violent assault.
3. 3
   The point at which something begins.
4. 4
   Alternative form of ground zero.

Proper noun

1. 1
   The site of the former World Trade Center towers in New York City destroyed on 11 September 2001.