The "ground zero" term originates from the US Army, defined shortly after the war as the point on the ground located directly beneath the explosion. The bombing target was the Aioi Bridge, but the bomb actually exploded a few hundred meters away from the bridge.
The ground zero in Hiroshima is sort of a Km. 0 of the suffering... but it passes almost unnoticed by the thousands of tourists visiting the Hiroshima Peace Memorial. I searched for it and I found it on a small street nearby Genbaku Dome, where it is marked discreetly - the brown stone marker on the left of the photo.
It is a modest monument and if you're not careful, you can pass by without noticing... especially since next to it is a busy parking lot, with vivid signage, like everywhere in Japan:
EXIF info:
Nikon D90
Lens: VR 18-55mm F/3.5-5.6G
Focal Length: 32mm
Aperture: F/7.1
Shutter Speed: 1/40s
ISO Sensitivity: ISO 640