A 50,000 square foot underground bunker, nuclear hardened and protected from chemical, biological and nuclear agents. It was built in ~1962 to support microwave and underground coaxial cable circuits that linked military bases throughout the country and also carried civilian traffic. This site is still active and operational with no plans to get rid of it in the near future.
The facility has extremely thick poured concrete ceiling and walls reinforced with many tons of steel rebar, the entire structure is buried approximately 60 feet underground. An enormous and complicated HVAC system was required to cool the electronic equipment inside and it was designed to filter out chemical, biological and/or radioactive contamination. The building uses a series of blast doors on all entrances and air intakes to protect the interior from the overpressure associated with a nuclear detonation. Upon detecting the radiation from a nuclear detonation, the gamma ray detector (pictured in video) topside would cause the system to seal up in anticipation for the blast that would follow some seconds after the gamma rays were detected.
At the bottom of the main staircase employees were faced with a multi-ton blast door which would mechanically interlock in the event of a nuclear detonation. Any employees unlucky enough to be caught outside and lucky enough to have somehow survived the blast would enter the facility through the blast door air lock system and then proceed directly into a decontamination shower.
The facility is still in operation but most of the mechanical and older telco equipment was retired in place in the late 80s early 90s and much newer telco equipment was installed over the years.