What underground nuclear bunker near Blue Ridge Summit, Pennsylvania known as "underground Pentagon" became operational in 1966?
Deep Underground Command Center:  Other contemporary underground installations did see upgrades, such as the 1953 Site R which was "hardened further to about 140 psi blast resistance by 1963" , or completion, such as the NORAD's Canadian bunker in 1963, and NORAD's Combat Operations Center & Space Defense Center in the Cheyenne Mountain bunker became operational in 1966. Raven Rock Mountain Complex: The Raven Rock Mountain Complex (RRMC) is an American military installation with an underground nuclear bunker near Blue Ridge Summit, Pennsylvania, at Raven Rock Mountain that has been called an "underground Pentagon."
Raven Rock Mountain Complex