Do I think there is underground facilities most likely for all nations and most would be very concerned being in them. Another aspect is on the secret space program and transportation of equipment regarding payloads using conventional rockets?
Most are likely within and circle of protection. Like any secure facilities above or underground facilities have strict procedures to get into as most would come under a military organisation and even private contractors are vetted through agencies as the does the Chinese , French, Germans,North Koreans , Russians any other groups. Also industrial espionage would be another angle that would watch out for. Example Lazar Case and civilians on bases. Just imagine what it would be like to penetrate these facilities with conventional forces?
On another topic of underground facilities sewage tunnels and great funny horror movie the killer fat blob Engineers may have just found the father of all fatbergs .
coincidentally I saw the Fatberg story on the BBC yesterday and they seemed to imply that the problem was cause by residential waste, as in people at home putting oil down the sink or toilet, but I suspect that it was at least in part caused by commercial food outlets, like takeaways and restaurants. I have emailed them (the BBC about this if I get a reply I write here how they respond.) London has miles and miles of sewers but they were not designed for the volume of fat being disposed of, it is easy to forget how few calories people used to live on. We also have the Tube(metro), electricity, gas and water pipes under our feet but the majority of the time you don't give them a second thought. Out of sight out of mind.
Regarding the secret space program and the transportation of equipment payloads using conventional rockets, I think rockets are very expensive. I believe a Super Gun is much more economic, they have been able to fire into space for a long time now. There was a Canadian designer who developed the technology in the 60s I think.
Super High Altitude Research Project
If they found a way to make it a bit less noisy and visible they could have such a thing quite close to population centers and nobody would notice.
I am not sure how badly a 'silencer' would hurt performance so maybe building it underground would have advantages in terms of concealment, especially if it could be fired from underground, from under water or even from under the seabed.
The Germans had something called the V3 which was basically a part subterranean multi stage super gun in the 1940s:
If they had completed it they could have fired on London 24/7
Operation Paperclip (and the Soviet equivalent) may have included scientists that worked on these projects.
But I think it should also be taken into consideration that secret development of better guns and better projectiles could mean that the area needed to house such a contraption is much smaller than we currently think.
For example there is an Australian multi-barrel electrically fired gun that can potentially fire a million rounds per minute. 'metal storm'
Maybe I was more influenced by Stingray than I thought