The Vatican unseals the archives of history's most contentious popes on Monday, potentially shedding light on why Pius XII stayed silent during the extermination of six million Jews in the Holocaust.

Basilica di San Pietro in Vatican Photo: VCG

Two hundred researchers have already requested access to the mountain of documents, made available after an inventory that took more than 14 years for Holy See archivists to complete.

Award-winning German religious historian Hubert Wolf will be in Rome on Monday, armed with six assistants and two years of funding to start exploring documents from the "private secretariat" of the late pope.

Wolf, a specialist on the relationship of Pius XII with the Nazis, is anxious to discover the notes of his 70 ambassadors - the pontiff's eyes and ears during his time as head of the Catholic Church between 1939 and his death in 1958.

There should also be records of urgent appeals for help from Jewish organizations, as well as his communications with the late US president Franklin D. Roosevelt.

The unsealed archives additionally cover a post-World War II era in which writers were censored and some priests hounded.

The Vatican first published the essentials covering the Holocaust four decades ago, an 11-volume work compiled by Jesuits.

But some crucial pieces are still missing, including the pope's replies to notes and letters - for example, those about Nazi horrors.

The Jesuits already published "documents the pope received about the concentration camps, but we never got to see his replies," Wolf said in an interview.

"Either they do not exist, or they are in the Vatican," he told AFP.

Historians have already examined the 12 German years of Eugenio Pacelli, the future pope's real name which he used while posted there as the Holy See ambassador in 1917-29.

There, he witnessed the rise of Nazism, then returned to Rome to become the right-hand man of his predecessor Pius XI, elected in 1922.

Past archives have revealed exchanges in which he was alerted about the extermination of European Jews once he himself became the pope.

"There is no doubt that the pope was aware of the murder of Jews," Wolf said.

"What really interests us is when he learned about it for the first time, and when he believed that information."

On December 24, 1942, Pius XII delivered one of history's most debated Christmas radio messages.

Buried in its long text was a reference to "hundreds of thousands of people who, without any fault of their own and sometimes for the sole reason of their nationality or race, were doomed to death or gradual extermination."

Was his message - delivered in Italian and aired just once, and which never explicitly mentioned either the Jews or Nazis - heard and understood by German Catholics?

Those who rise to the pope's defense note that Pius XII was a former diplomat who was trained in prudence, anxious to remain neutral in time of war, and concerned about being able to shield Catholics from the unfolding devastation.

He simply could not be any more explicit, Pius XII's supporters say. Historians estimate the Church hid around 4,000 Jews in its Roman institutions during the war.