Sedevacantism
A small group of traditional Catholics rejects the reforms of the Second Vatican Council, and in some circles this rejection extends even to the validity of the post-Council Popes themselves; in their view, Pope Pius XII was the "last true Pope."
A branch of sedevacantists, adherents of the thesis, who are also known as Sirianists to adversaries of the Thesis, also believe that John XXIII and the official popes are antipopes, while Siri reigned as the suppressed head of the Catholic Church until his death in 1989. However, unlike in historical antipope disputes, Siri has never claimed to be the actual Pope nor questioned the validity of the Papacy of John XXIII, Paul VI, John Paul I or John Paul II.
Read more about this topic: Siri Thesis