This complex, known as the Manhattan Project (or MP) was housed in a series of underground caves designed to protect the Union government if the Confederacy started to win the United States Civil War (1861-1865). After the war the project was abandoned until it was later revised and expanded to 300 acres during World War One (1914-1918). Since that time it has been expanded several times to now include over 700 acres in a huge underground city within a city. It is located deep within the bedrock, over 300 feet below the park. This massive city complex is now designed to safely house up to 4,000 central Government personnel in the event of a nuclear strike. It presently houses all United States government UFO activities and was the home for Nazi Adolf Hitler until his death in 1956. This Super-secret location is the site of the main Emergency Government War Headquarters - the hub of the Country's alternative seat of power outside Washington, D.C. http://gothamist.com/2017/01/23/its_not_a_bad_idea_really.php
You had me until you said Hitler. Good conspiracy theory though.