CRUX has a online picture album from inside the Vatican archives. It’s not the stuff of Dan Brown, but still rather fascinating:
The archives, or Archivum Secretum Apostolicum Vaticanum, contain historical records chronicling intriguing historical events. Its contents, once plundered by Napoleon and moved to Paris, span 12 centuries.
And from the page of further explanation:
Secretum, the Vatican says, translates more accurately to “personal” than to “secret” and refers to the private letters and historical records of past popes. In fact, the archives haven’t been secret since 1881, when Pope Leo XIII opened them up to scholars.