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Good question.
It was discussed in detail here on FR. That discussion included how collapse of the spherical explosion void caused fracturing and collapse of a rock column above it -- reaching to the top surface of Mount Mantap. That created a "chimney" through which any subsequent testing within the mountain would be almost guaranteed to "vent" radioactive gases and particles to the atmosphere.
Such "venting" would greatly displease NK's huge neighbors And, it would provide other nations -- with the technical capabilities of collecting & analyzing the effluvia -- with invaluable intel re NK's weapon structure and performance...
The discussions also included OHI evidence of the "slump" in the upper surface of Mantap, as well as avalanches on the mountainsides.
And, of course, seismographs at several locations detected the internal collapse following the explosion.
There are, reportedly, ways to minmize such collapse and venting -- but, they, apparently, were beyond the scope of NK's scientific/engineering expertise.
So, KJU wound up with a wrecked test site, and all he could do was make a propaganda show of "destroying" the already-destroyed site.
Paper Tiger...
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Yet -- as you mentioned -- little or none of the above was covered by the DSM...
Are you surprised? There are pitfully few in the entire Deep State "journalism" industry who could even spell. "effluvia" -- much less comprehend its geopolitical and intelligence significance.
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(Probably as good a non-technicalpublic source as any is 38 North...)
Collapse and Earthquake Swarm After North Korea’s3 September 2017 Nuclear Test