UPDATE: North Korea has just announced their fifth and reportedly biggest nuclear test. The test was carried out at an underground facility at the Punggye-ri test site. According to the North Korean government, the aim of the test was improvement and miniaturization of nuclear warheads.
Following satellite imagery, accessed via Spaceknow Analytics, captures the Punggye-ri test site between 2011 and 2016. The images confirm that the facility expanded over time.
Since the test was carried out underground, it is harder to monitor changes in the area from space and determine the veracity of North Korean claims.
Still, the images show numerous signs that the North has carried out various underground construction works, especially in the northern part of the site.
The most recent image from July 2016 shows new earth spoils in the northern part as well as completion of a small facility (the facility is now visibly fenced) next to the main buildings in the center.