6. Vatican Secret Archives, Vatican

If you have ever wondered about the secrets relating to the Catholic Church, you might have to keep waiting because the Vatican Secret Archives is under tight security. The archives contain documents that date as far back as the 8th century, including papal account books, a letter from Michelangelo to Pope Julius II, and a letter from Mary Queen of Scots written before her execution. 

The mysterious vault is located underground, and it has about 53 miles worth of shelves. No one is allowed to enter besides researchers who are given special permission, but even they have limitations to what documents they can look at. Some secrets will never be revealed.