science/geophysical sites/publications, especially since their last test in September, the mother of all their tests to date, after which the mountain (visibly) collapsed, which was even predicted, because of "tired mountain" syndrome, as a result of their other (5) tests. They only have/had one test site.

All you gotta do is Google it.

From the clickable link below:

According to sources, people from the Punggye-ri nuclear test site have been banned from entering Pyongyang since the test due to the possibility of being radioactively contaminated.[13] According to the report of defectors, about 80% of trees died and all of the underground wells dried up in the site after the sixth nuclear test.[14]

On 3 and 23 September 2017, earthquakes which seem to be collapses of tunnels were detected with magnitude of 4.1 and 3.6 respectively.[1] A 17 October 2017 study published by the US-Korea Institute at Johns Hopkins University suggested the most recent test had caused "substantial damage to the existing tunnel network under Mount Mantap".[15][16]

On 30 October 2017, in testimony before the South Korean parliament, the director of South Korea's Meteorological Administration warned that "further tests at Punggye-ri could cause the mountain to collapse and release radioactivity into the environment."[16] Likewise, Chinese scientists warned that if the mountain collapsed, nuclear fallout could spread across "an entire hemisphere."[16]

On 1 November 2017, Japanese TV station TV Asahi reported that according to unconfirmed reports, several underground tunnels collapsed at the test site on 10 October 2017.[17] An initial collapse was said to have killed 100 workers, with another 100 rescuers killed in a second collapse.[18][19]

On 20 April 2018, the North Korean government announced that it would suspend nuclear tests and shut down the Punggye-ri nuclear test site.[20]