Recent satellite images show busy activity at North Korea's nuclear test site, reports 38 North, which is run by John Hopkins University's School of Advanced International Studies.
The images showed large stacks, and what could be mine ore carts, suggesting tunnel work. The resolution of the images wasn't clear enough to show what exactly the people were doing at the Punggye-ri test site, however.
In the report: "It is clear that North Korea is ensuring that the facility is in a state of readiness that would allow the conduct of future nuclear tests, should the order come from Pyongyang."