Question: What underground nuclear bunker near Blue Ridge Summit, Pennsylvania known as "underground Pentagon" became operational in 1966?
Context: 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."  The bunker has emergency operations centers for the United States Army, Navy, and Air Force.  Along with Mount Weather Emergency Operations Center in Virginia and the Cheyenne Mountain Complex in Colorado, it formed the core bunker complexes for the US Continuity of Government plan during the Cold War to survive a nuclear attack. The Federal Property and Administrative Services Act of 1949 after the 1947 reorganization for establishing the National Military Establishment was passed regarding reductions of US military bases, forts, posts, and stations.  The subsequent 1950s buildup for the Cold War (e.g., during the Korean War) resulted in extensive installations such as the widespread number of Permanent System radar stations and Semi Automatic Ground Environment (SAGE) control centers.  By 1959, plans for extensive numbers of Cold War installations were cancelled (e.g., DoD's June 19, 1959, Continental Air Defense Program reduced the number of Super Combat Center underground nuclear bunker to 7) and in 1958, US Intercontinental Missiles (ICMs) began to replace Strategic Air Command bombers.  From 1960–1964, the Eisenhower and Kennedy Administrations closed "574 U.S. military bases around the world", particularly after President John F. Kennedy was briefed after his inauguration that the missile gap was not a concern. Monterey Pass is a mountain pass near Blue Ridge Summit, Pennsylvania, and the Mason–Dixon line.  The saddle area lies near Happel's Meadow between Monterey Peak (1,663 ft) and Clermont Crag (1627 ft).  It was the site of the July 1863 Fight at Monterey Pass during the Retreat from Gettysburg. The Deep Underground Command Center (DUCC) was a United States military installation that was proposed on January 31, 1962 , to be "a very deep underground center close to the Pentagon, perhaps 3,000-4,000 feet down, protected to withstand direct hits by high-yield weapons and endure about 30 days in a post-attack period" .  The DUCC would have been built as "an austere 50-man … or an expanded 300-man version (with the former built to permit expansion into the latter, if desired)" .  It was designed to withstand multiple direct hits of 200 to 300 Megaton weapons bursting at the surface or 100 MT weapons penetrating to depths of 70-100 feet.  Based on Strategic Air Command's Deep Underground "Support" Center (DUSC) planned near the Cheyenne Mountain nuclear bunker , the DUCC plan was recommended to President John F. Kennedy for fiscal year 1965 funding shortly before his assassination , but the underground DUCC, SAC's DUSC, and NORAD's Super Combat Centers were never built.  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. A nuclear bunker buster, also known as an earth-penetrating weapon (EPW), is the nuclear equivalent of the conventional bunker buster.  The non-nuclear component of the weapon is designed to penetrate soil, rock, or concrete to deliver a nuclear warhead to an underground target.  These weapons would be used to destroy hardened, underground military bunkers or other below-ground facilities.  An underground explosion releases a larger fraction of its energy into the ground, compared to a surface burst or air burst explosion at or above the surface, and so can destroy an underground target using a lower explosive yield.  This in turn could lead to a reduced amount of radioactive fallout.  However, it is unlikely that the explosion would be completely contained underground.  As a result, significant amounts of rock and soil would be rendered radioactive and lofted as dust or vapor into the atmosphere, generating significant fallout. USS "Blue Ridge" (LCC-19) is the lead ship of the two "Blue Ridge"–class command ships of the United States Navy, and is the command ship of the United States Seventh Fleet.  Her primary role is to provide command, control, communications, computers, and intelligence (C4I) support to the commander and staff of the United States Seventh Fleet.  She is currently forward-deployed to U.S. Navy Fleet Activities, Yokosuka in Japan, and is the third Navy ship named after the Blue Ridge Mountains, a range of mountains in the Appalachian Mountains of the eastern United States.  "Blue Ridge" is the oldest deployable warship of the U.S. Navy, following the decommissioning of  in Pearl Harbor on 14 August 2014.  "Blue Ridge", now the U.S. Navy's active commissioned ship having the longest total period as active, flies the First Navy Jack.  "Blue Ridge" is expected to remain in service until 2039. Blue Ridge Summit is an unincorporated community and census-designated place in Franklin County, Pennsylvania, United States, southwest of Gettysburg in the central part of the state, adjoining Pennsylvania's southern border with Maryland.  It is less than 3 mi east of Pen Mar, Maryland.  The population of Blue Ridge Summit was 891 at the 2010 census. The Hack Green Secret Nuclear Bunker is a former government-owned nuclear bunker located at Hack Green, Cheshire, England. The Deep Underground Support Center (DUSC) was a Strategic Air Command nuclear bunker proposal in 1962 for "a hardened command post...to withstand a 100-megaton weapon with a 0.5 n.m. CEP".  Favored for a mine near Cripple Creek, Colorado (west of the Cheyenne Mountain nuclear bunker started in 1961), the DUSC was to be 3500 ft deep and be "able to accommodate some 200 people for [30 days] to handle the large volume of data processing and analysis required for strike assessment, as well as follow-on strike and other decisions."  Cost estimates for the SAC Control System facility increased to $200 million, and when the operational year slipped from 1965 to 1969, SAC decided in 1963 "for a long-endurance, all airborne concept instead" (Wainstein), and the JCS and OSD concurred with the DUSC project cancellation. Maryland Route 550 (MD 550) is a state highway in the U.S. state of Maryland.  The state highway runs 24.43 mi from MD 26 in Libertytown north to Pen Mar Road in Fort Ritchie.  MD 550 runs southeast–northwest across central Frederick County, connecting Fort Ritchie in the northeastern corner of Washington County and Libertytown with the towns of Thurmont and Woodsboro and the smaller communities of Creagerstown and Sabillasville.  South of the highway's junction with U.S. Route 15 (US 15) in Thurmont, the state highway passes through the wide valley of the Monocacy River; to the north, the highway passes along the northern edge of Catoctin Mountain and crests South Mountain near Blue Ridge Summit, Pennsylvania.
Answer: Raven Rock Mountain Complex