30 June 1946, NEW YORK TIMES, pg. 3:
"The most instructive fact at Nagasaki was the survival, even when near
ground zero (the spot below which the bomb exploded) of the few hundred
people who were properly placed in the tunnel shelters, though unoccupied,
stood up well in both cities."
(The OED's first citation for "ground zero" is the NEW YORK TIMES of July 7, 1946.)

