Vatican Secret Archives
Housed in Vatican City, these documents contain state papers, papal account books, and correspondence. The use of the word “secret” in this case is more akin to the word ‘private’ rather than ‘hidden.’ Still, good luck trying to enter these premises. There are 53 miles of shelving which contain around 35,000 volumes. Only qualified scholars can apply for an entry card into the site. They still need an introductory letter from a recognized institute of research.