The Vatican Secret Archives
The Vatican’s Secret Archives are inaccessible; the actual place, that is, but if you’re an accredited researcher, you can view any document you like. The Archives preserve documents that go as far back as the 8th century and its shelves stretch a mind-boggling 85 kilometers (53 miles) holding treasures such as Henry VIII of England’s petitions for divorce from Catherine of Aragon; documents relating to the trial of Galileo in 1633 on charges of heresy; letters from Michelangelo regarding late payments for his work; a 1521 bull of excommunication of Martin Luther and a letter from Mary, Queen of Scots, written while awaiting her execution. After two thousand years of history, you would have such things. The only documents you can’t access are those that are not yet 75 years old (in order to protect diplomatic and governmental information). Indexes are available for people who want to see if a document exists in the archives, but your presence is neither required nor wanted.