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.
i don't know