ground zero

US /ˌɡraʊnd ˈzɪr.oʊ/
UK /ˌɡraʊnd ˈzɪr.oʊ/
"ground zero" picture
1.

the point on the earth's surface directly above or below a nuclear explosion or other large bomb

:
The city center was declared ground zero after the bombing.
Scientists studied the effects of the blast at ground zero.
2.

the site of the World Trade Center in New York City, destroyed in the terrorist attacks of September 11, 2001

:
A memorial now stands at Ground Zero in New York City.
Many people visit Ground Zero to pay their respects.
3.

the very beginning or most basic level of something

:
We have to go back to ground zero and rethink our entire strategy.
His understanding of the subject is still at ground zero.