The 25th Air Division and Seattle Air Defense Sector complex were built in the late 1950's and became operational in January 1960 on McChord Air Force Base, Washington. The facility complex known as the "blockhouse" housed the world's two largest digital computers at the time. Radar sites around the Pacific Northwest feed information into the computers, which solved problems of position, speed, altitude, and armament for interceptor directors, who in turn guided pilots to targets. (Courtesy photo)