I was going over new Google Earth imagery of the east coast and saw an interesting new underground facility (UGF) near Tanchon (40.409103°, 128.842130°). Based on Google Earth imagery, it appears to have been built sometime between 2015-7-8 and 2017-11-10. Based on @Planetlabs imagery it appears construction was carried out in the latter half of 2016, though I cannot say with precision owing to image quality.
This UGF is constructed in an abandoned railway tunnel, construction of which was halted at a time that pre-dates commercial satellite imagery (i.e. the tunnel was really old). Utilizing the plan for the original tunnel we can speculate about the size and shape of the UGF in this hill, but there is no evidence to conclude that the tunnel has not been altered as it was converted to meet its new purpose.
There are several other abandoned railway tunnels throughout the DPRK, but this is the first of which I am aware that has been converted to an alternate use.
It may be military related, but not necessarily. It could be a mushroom farm or it could be an underground document storage facility (both of which can be found all over North Korea)–or something else.
Interestingly, the road running into it has not been renovated all that much, and as of yet, I cannot see any power transmission lines running here.
