ground zero

Ground zero is the point on the earth's surface directly below or above a nuclear explosion, where destruction is most complete.

The term extended to mean any site of catastrophic destruction or, more loosely, the starting point or central focus of any significant activity: Let's start from ground zero.

Since 2001, Ground Zero commonly refers to the World Trade Center site in New York City.

The investigation returned to ground zero—the warehouse where the fire began. (Capitalized Ground Zero typically refers to the World Trade Center site.)