Historians have begun combing the archives of the world's most contentious pope, hoping to glean why Pius XII stayed silent during the Nazi extermination of Jews in the Holocaust.

More than 200 researchers have applied for permission to begin poring over the millions of letters and documents the Vatican had kept under lock and key.

The historic moment was preceded by decades of controversy and debate about why the pontiff, who headed the Catholic Church from 1939 until his death in 1958, never spoke out about the murder of six million Jews in Nazi concentration camps across Europe during World War II.

"The church is not afraid of history," Pope Francis declared when he chose to open one the Vatican's most painful moments up for world scrutiny a year ago.

"For millions of people, Catholic and Jewish, these archives are of enormous humanitarian interest," said Suzanne Brown-Fleming, international programmes director at the US Holocaust Memorial Museum in Washington.

Pope Pius XII files Vatican
A detail of the section of the archive dedicated to Pope Pius XII at the Vatican

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

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

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

German researcher Sascha Hinkel was one of those to have gained access to the trove through the summer months.

"This is a great opportunity," said Mr Hinkel, who is one of the researchers assisting the religious history scholar Hubert Wolf, an expert on Pius XII and the Nazis.

Mr Hinkel thinks it will take researchers about five years to answer the main questions, although the entire mountain of documents available for the first time "will occupy historians for at least 20 years".

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Secret Vatican archives: Was Pius XII 'Hitler's Pope'?

The Vatican's focus on transparency was symbolised a few days ago by the presence of reporters' cameras in the bunker of the central archive, officially known as the "secret archive" until as recently as last year.

The 85km web of dusty shelves includes a section dedicated to Pius XII, protected behind locked metal gates.

On this occasion, Archbishop Sargio Pagano, the central archives' manager, took out some tattered leaflets.

They were drawings and letters from German children thanking the pope for sending them first communion gifts in 1948.

Kept confined to the Vatican by the Nazis and then Italian Fascists, Pius XII was a German-speaking Italian aristocrat who witnessed Adolf Hitler's rise while posted as the Holy See's ambassador in Germany for 12 years.

The archives will give historians a chance to better understand Pius XII's reticence, which some view as unforgivable.

Others note that the church still saved around 4,000 Jews from certain death by hiding them in its Roman institutions and that he had to stay neutral to better shield Catholics from the unfolding devastation.

While opening the archives, the Vatican also appears to be shutting down talk of Pius XII's beatification, which is a first step toward canonisation backed by the German-born Pope Benedict XVI in 2009.

The idea triggered an outcry from Jewish groups.