Read Siri Thesis which states,

that actually Cardinal Giuseppe Siri was elected Pope, after the death of Pope Pius XII in 1958, but the newly-elected Pope (Gregory XVII, formerly Cardinal Siri) was *put under grave duress, threatened- prevented from taking the Papal Chair (i.e. publically proclaiming His Pontificacy) and replaced by Angelo Roncalli (John XXIII). There is evidence that in 1958, during the Conclave, the enemies of the Church were threatening "Siri" with mass destruction if he took the Chair of Peter (i.e. as stated "Cardinal Siri" after being canonically elected, accepted the papacy and chose the name Gregory XVII - the 5th column enemies within the Conclave walls themselves, then used ferocious and vicious, real threats against Pope Gregory XVII, if he publically were to announce he was the True Pope.) Pursuant to this thesis, all of the apparent 'popes' after Pope Pius XII were/are imposters (Anti-popes), as "Siri" was the true Pope.