Established in 1612 by Pope Paul V, the Vatican Secret Archive houses thousands of historical documents, manuscripts, and artifacts, some of which date as far back as 809 AD.
Despite bearing the word "secret" in its name, it didn't refer to the archives being unknown or hidden. Rather, the Latin would have been properly translated as "private" as the archive is regarded as the personal property of the current pope until his death or resignation, when the new pope will inherit the archive. It has since been renamed to the Vatican Apostolic Archive by Pope Francis in 2019.
Although unofficial, it is considered as one of the largest archives in the world, and has been strictly closed off from the general public until the late 19th century, when Pope Leo XIII gave access to researchers who wanted to study materials from the archive.
Today, gaining entry into the archive involves an arduous process and requires an applicant to fill out a form, attach a recommendation letter from a reputable scholar, and specify the document or artifact that they want to study. However, this is where we might have a Catch-22 problem, because we don't know much about what the archive holds.
Some items have been disclosed to the public and those include the Papal Bull issued by Pope Alexander VI granting King Ferdinand II and Queen Isabela I the right to rule over the New World, the documentation to Galileo's trial by the Inquisition, and letters from Abraham Lincoln and Jefferson Davis to Pope Pius IX.
(Image credit: The Vatican)