Just a little personal note: When I was in graduate school, I applied for a summer job at the NSA. Since, if hired, I would have been working on improving the SAC nuclear release codes, I needed to upgrade to a "Presidential" security clearance. (I already had a "secret" clearance from work I'd already done for NASA.)
Since I would only be working there during the summer break, the clearance process was "expiated," and I went to the NSA buildings outside Washington in early December for the required polygraph test and interview with a psychologist. The polygraph test was so boring that I fell asleep in the middle of it, and the psychologist seemed, um, well, inept.
But, all in all, they managed to come to the correct conclusion: They told me "thanks for asking, but no job."
The point in this story is two-fold: First, there are many levels of classification more restrictive than "Secret," and, second, access to data classified at those more restricted levels is much more controlled than access to data only classified as "Secret" or "Confidential."
In fact, I was told that the (approximately) twenty levels between "Top Secret" and "Presidential" were classified.
Judging from what has been disclosed in the various recent leaks, perhaps too much has been classified for no good reason, but, still, the higher levels of classification seem to be secure from leaks and not used without some objective, reviewed, reason.