Collections of documents pertaining to the affairs of the Holy See. Since apostolic times, the popes preserved writings of importance, but most of the material before Pope Innocent III (1161-1216) has been destroyed or has disappeared. Pope Paul V (1552-1621) established a central repository of documents of the Holy See. In 1881, Pope Leo XIII opened these archives to scholars for consultation. The Vatican Archives are connected with the Archivist School, which before 1968 was the annual Archivist Institute, first established by Pope Pius XI in 1923. The archives of the Holy See contain tens of thousands of documents and are the most important center of historical research in the world.