ground zero

noun

1. the point on the surface of the earth or water directly below, directly above, or at which an atomic or hydrogen bomb explodes.
2. (sometimes initial capital letters) the site of the former World Trade Center in New York City, destroyed in the September 11, 2001 terrorist attacks.
3. Informal. the very beginning or most elementary level: Some of the students are starting from ground zero.

noun

1. a point on the surface of land or water at or directly above or below the centre of a nuclear explosion
2. a scene of great devastation
3. (sometimes capitals) the name given to the devastated site of the collapsed World Trade Center towers in New York after September 11 2001