ground zero

noun3

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.

   • It's Christmas at ground zero / The button has been pressed / The radio / Just let us know / That this is not a test — 1986
2. •
   The location of any disaster or violent assault.

   • "This is ground zero," she said. "If regulators don't adopt a zero tolerance for violations of standards then what we will have are places that are going to have unacceptable environmental damage." — 2011
3. •
   (figurative) The point at which something begins.

   • James Meredith's forced admission was a milestone in upending the old order in America's most segregated state, a kind of race relations ground zero. — 1997