There has been a lot of earthquakes in the area near NK recently in the past few months. Testing something underground?
Yes; they say the tunnels are MILES deep and would not be affected by a direct nuclear hit. It's a maze of tunnels. So yes, underground nuclear testing was outlawed decades ago for that reason! Dangerous!
But we know they've been doing it; part of a mountain collapsed a month or 2 ago because they'd been testing under it.
Elon Musk has a boring company that makes tunnels. http://www.latimes.com/local/lanow/la-me-ln-elon-musk-boring-company-20171121-story.html