The Raven Rock Mountain Complex (also known as Site R, Alternate Joint Communication Center, Alternate Military Command Center, and colloquially ‘The Rock’) was first conceptualized in 1948 during the early years of the Cold War.  When the Soviet Union successfully tested their first nuclear weapon in 1949, the concept was made a top priority.  In 1950 President Harry S. Truman approved the project and construction started a year later.  The location of Raven Rock Mountain in Pennsylvania was made due to its close proximity to both Washington D.C. and Camp David, near Thurmont, Maryland.  The site was operational by 1953. 

Citations: 
"Raven Rock Mountain Complex (Site R) | Public Intelligence." Public Intelligence. Web. 6 Oct. 2015. <https://publicintelligence.net/raven-rock-mountain-complex-site-r/>.
 
"7th Signal Command Theater." 7th Signal Command Theater. Web. 6 Oct. 2015. <http://www.7sigcmd.army.mil/>.
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The Alternate National Military Command Center was blasted out of Raven Rock Mountain, about 6 miles north of Camp David on the Pennsylvania-Maryland border. This huge underground bunker was built in around-the-clock (24/7) shifts between 1950 and 1953 as a backup Pentagon and communications center should Washington, DC be destroyed. From http://cryptome.info/0001/anmcc.htm