Pictures of nuclear weapons themselves (the actual casings) were not made public until 1960, and even those were only mock-ups of the "Fat Man" and "Little Boy" weapons dropped on Japan—not the more powerful weapons developed more recently. Diagrams of the general principles of operation of thermonuclear weapons have been available in very general terms since at least 1969 in at least two encyclopedia articles, and open literature research into inertial confinement fusion has been at least richly suggestive of how the "secondary" and "inter" stages of thermonuclear weapons work.