Research by Chinese geologists suggests that the mountain above North Korea's main nuclear test site has likely collapsed, rendering it unsafe for further testing and requiring that it be monitored for any leaking radiation. Nuclear explosions release enormous amounts of heat and energy, and the North's largest test in September was believed early on to have rendered the site in northeastern North Korea unstable. minutes after the explosion, was "an onsite collapse toward the nuclear test center," while those that followed were an "earthquake swarm" in similar locations. North Korea's nuclear tests are of special concern to Beijing, since the test site near the town of Kilju is less than 100 kilometers (60 miles) from the border with China. The region isn't one where earthquakes naturally occur and no quakes were detected after the five smaller nuclear tests North Korea has conducted since 2006.

Source: ABC News April 25, 2018 23:26 UTC