54°14'41.59"N 58° 4'54.31"E
Map
This was believed to be a Russian Nuclear Shelter in Yamantau Mountain all the way up to 2009. You can find lots of pictures now that it isn't active. It looks more like a massive mine than anything else. You can even find a Youtube video showing some pictures inside:
https://www.youtube.com/watch?v=tiqATtRWawQ
For some strange reason, this guy decides to show himself holding a phone for 30 seconds. If you're impatient, just skip ahead to the 50 second mark, and you will see pictures taken of the crumbling inside.