Secretive Russian town of Mezhgorye

There is a town in Russia, called Mezhgorye, which is barred to anyone, except for those believed to work at the highly secretive Mount Yamantaw site, thought by many to be either a ultra-secret research facility, or be nuclear. Founded in 1979. The town is at the foot of the mountain, the highest in the Urals, at 5,381ft, where US satellites have recorded excavation projects, on a huge scale, though repeated enquiries about the nature of operations, at this maddeningly closely watched community, have been met by bland responses, such as it being a mine of some kind, or a repository for Russian treasures, and even as somewhere for government, in case of disaster, but nobody knows.