1960’s largest vacuum chamber was...
Chamber A is now the largest high-vacuum, cryogenic-optical test chamber in the world, and made famous for testing the space capsules for NASA’s Apollo mission, with and without the mission crew. It is 55 feet (16.8 meters) in diameter by 90 feet (27.4 meters) tall. The door weighs 40 tons and is opened and closed hydraulically.
Oh wait, wasn’t it your supposed secret military underground complex....ahhhhh...riiiiiiight.
Paging Art Bell