During the tense Cold War years from 1953 to 1979, the United States Army built and operated hundreds of Nike missile firing batteries in the United States to protect against a Russian air assault that never came. When the Cold War ended most of these installations were stripped and simply left to rot, never having fired a single missile, as is the case with Nike Missile Site SF-51 whose abandoned concrete structures have proven more useful to graffiti artists than they ever were for the military.
These missile sites were created as the last line of defense against Soviet bombers. Nike Missile Site SF-51 located on Sweeney Ridge in Pacifica California was just one of 11 such sites erected to guard the Bay Area with their super-sonic missiles ready at the push of a button. During its operational height the base was equipped with advanced radar that could detect incoming enemies and deploy Nike missiles in seconds. Luckily the button never had to be pushed and the entire facility proved nothing but an effective deterrent. With the introduction of projectiles with intercontinental range, the Nike program became redundant and the cinderblock structures on the Pacifica site were left to crumble in 1974.
However, even after being stripped of any dangerous or valuable equipment, many of the blocky buildings still remain today. While there are a couple of informational signs that might appeal to history buffs, the area seems to be of most interest to graffiti artists who have all but covered the remaining walls with spray art. The site is accessible by trail on foot, bicycle, or horse. The site is currently a part of the Golden Gate National Recreation Area (GGNRA).
Update as of October 2019: Much of the site has been demolished.