North Korean nuclear tests set off 1,399 earthquakes: Study

North Korea's six underground nuclear tests at Mount Mantap from 2006 to 2017 reactivated dormant fault lines and triggered 1,399 small earthquakes, even years after the last test, a study found. "Instead of being strongest immediately after the explosion and then fading, the activity increased over several years," a seismologist said. Notably, the area is not naturally seismically active.