A train rushed through the snow of a Polish winter. Its destination: the Warsaw Ghetto. Its passengers: a group of terrified Jews. Suddenly, a Nazi guard threw a three-year-old child off the train and into the snow. Its mother jumped off the train, too, desperate to save her child. It was too late. She arrived at the ghetto desperate and mentally ill.

Thistale of mother and child is just one of the devastating stories we now know about the more than 400,000 Jews who were packed into the Warsaw Ghetto. But it wouldn’t be known at all if not for a secret archive assembled by a group of Jews inside the sealed-off walls of the 1.3-square-mile area they were forced to live in beginning in 1940.

Between 1940 and 1943, these residents amassed a rich trove of documents and testimonies designed to tell the ghetto’s stories. And though thousands of pages of their Holocaust archive survive, even more may still be buried beneath the streets of Warsaw.

The Ringelblum Archive, as it is known, was the work of Emanuel Ringelblum, a Polish social worker who also established a soup kitchen, welfare programs and even a society for the advancement of Yiddish culture within the ghetto. Before Warsaw’s Jewish residents were forced to live behind ten-foot walls topped with barbed wire, Ringelblum had been an aid worker for the American Jewish Joint Distribution Committee, which helped Jews in Eastern Europe.

As starvation, disease and cold began to kill large numbers of people within the densely packed ghetto walls, Ringelblum became obsessed with documenting the complete reality of the lives of Jews at the time. Together with a group of writers, rabbis, social workers and others who met surreptitiously on the Sabbath, he assembled letters, artwork, posters, data and even packaging from theworkshops that produced consumer products within the ghetto.

The work was covert and feverish, especially as it became clear that time was running out for Ringelblum and his colleagues, whom he called “Oneg Shabbat,” a Hebrew phrase that means “joy of the Sabbath.” In 1942, Nazis had begun deporting Jews from the ghetto, and between July and September, theytook about 265,000 Jews to the Treblinka death camp. Another 35,000 Jews were killed inside the ghetto during the deportations.

As conditions deteriorated, the group stepped up their work. Eventually, wrote Ringelblum, “We reached the conclusion that the Germans took very little interest in what the Jews were doing amongst themselves.”

This relative freedom meant that Ringelblum and his co-conspirators could interview ghetto residents and assemble their archive without attracting much suspicion—although continual raids and deportations interrupted their work.

“There was missing the quiet atmosphere that is needed for a task of such size and dimensions,” Ringelblum later recalled of the conditions under which the archive was compiled.

In 1942, the members of Oneg Shabbat were contacted by Szlamek Bajler, a Polish Jew who had escaped the Chełmno extermination camp. Under the pseudonym Jacob Grojanowski, Bajler gave detailed testimony of the atrocities he had seen at Chełmno to Ringelblum and his associates. They smuggled thereport to the Polish resistance. Their hope was that the report would make its way both to Germany and to the Polish government in exile, but it’s unclear what happened after it was smuggled out of the Warsaw Ghetto.

Then, in 1943, rumors circulated that the ghetto would be liquidated. As a growing resistance prepared to fight back against the Nazi soldiers who had been sent to deport the ghetto’s residents to death camps, Ringelblum and his colleagues scrambled to protect their archive. They hid the archive in metal boxes and milk cans and buried them in three locations just one day before the beginning of the nearly month-longWarsaw Ghetto Uprising.

His precious archive safe, Ringelblum escaped to the non-Jewish part of Warsaw. He was later apprehended and taken to the Trawniki labor camp, but escaped and went into hiding in Warsaw again. Then, in 1944, someone reported his hiding place to the authorities. He was taken to Warsaw’s Gestapo headquarters along with his family and murdered.

All but three of the Oneg Shabbat group were killed in the Holocaust, but their work outlived them. In 1946, ten metal boxes were unearthed; conservators were able to restore most of the pages, though they were badly waterlogged. In 1950, the second part of the archives was found. Today, the archive of about 35,000 pages is hosted at the Jewish Historical Institute in Warsaw.

But a third part still remains buried. Though it’s been sought ever since, it has never been found.