According to Dr. Jeffery Lewis, at Armscontrolwonk.com, the site became publicly known in 1998 (the US government knew about it since 1996) and was constructed at the same time as another underground site, Kumchangni (40° 7' 8 "N 125° 8' 32"E). Since such a site could possibly violate the bilateral 1994 Agreed Framework, North Korea didn't say a word about the place. After the site became public, the US managed to send a delegation to visit it. They didn't find much besides an oddly designed underground facility.
The debate about what exactly is going on at Hagap continues to this day. It has been speculated that the site is used for nuclear materials storage, centrifuge production, or even just a large secure warehouse for archival materials. However, the connection with Kumchangni still remains. There is a third site too, at Yeongjeo-ri (Ryanggang Province), but little is known about it.
North Korea's nuclear program has been ramping up since the final days of Kim Jong-il, and Kim Jong Un seems to be increasing that pace. In the light of that, it is disconcerting to see fairly substantial new activity happening at Hagap.
(Click image for larger view.)
At the site you can see a large mound of new rock debris which has been piled up in the pond below the main entrance. You can also see an increase in the number of small buildings and debris that are in the small valley.
Here are some closeup images:
This one shows the debris mound.
This image shows the extended activity area with new buildings.
There is also an odd collection of towers nearby. They look like electrical transmission pylons, but are clustered together into three groups. There are no visible power lines either. It's possible these were laid out when Hagap was originally constructed to provide power, but haven't been needed since the site was largely abandoned. Or they could be some kind of radar/communication array. The towers are located around a bend in a river with small hills on either side, those hills have an anti-aircraft artillery battery stationed on each one. If you'd like to study the area further, it can be found here 40° 4' 50"N 126° 6' 35"E.
--Jacob Bogle, 1/12/2016