(November 13, 2013 at 7:24 pm)Cthulhu Dreaming Wrote: That isn't fallout. What force is driving the contaminated material into the atmosphere from where it would fall out?
The roof collapse of the cavity compresses the air inside, which then erupts through breaches in the roof of the cavity, entraining the radioactive material from inside the cavity and expel them into the atmosphere.
I don't know what fraction of underground test cavities collapsed. But aerial photos of the Nevada test site certainly shows very large numbers of seriously fragmented collapse depressions over test cavities.