In 1896, Agnes Smith Lewis and Margaret Dunlop Gibson, the identical-twin daughters of a worldly Scottish widower, returned to Cambridge from a trip to the Middle East bearing pages from several ancient Hebrew manuscripts that they had purchased from a Cairo bookseller. They showed the parchment leaves to a fellow Cambridge University scholar named Solomon Schechter, who was startled to discover among them an original copy of the Hebrew proverbs of Ben Sira, a second-century B.C. Hebrew book of wisdom. He wrote to his friend Adolf Neubauer, a like-minded librarian at Oxford, with the news. Neubauer replied two weeks later, saying that he couldn’t quite make out Schechter’s postcard but that he and his assistant, Arthur Cowley, had just—“coincidentally”—discovered nine pages of Ben Sira at Oxford. Of course, there was no coincidence about it. Schechter’s postcard had sent Neubauer on a hunt through his own Cairo trove.

Enraged, Schechter set off to Fostat (Old Cairo), where the manuscripts had been found, eventually making his way to the Ben Ezra synagogue—the site, according to legend, where baby Moses had been found in the reeds. Deep within the building, in a hidden repository called a genizah (from the Hebrew word ganaz, meaning to hide or set aside), Schechter uncovered more than seventeen hundred Hebrew and Arabic manuscripts and ephemera.

In 1897, Neubauer and Cowley beat Schechter to publication of the Ben Sira discovery. But Schechter did them one better, and made it back to England with the Genizah mother lode. He and his patron Charles Taylor, who was then Master of St. John’s College, donated the fragments to Cambridge in 1898. They published their account of the discovery in 1899 and facsimiles of the documents in 1901. Schechter and Neubauer would not exchange any more friendly postcards.

Oxford and Cambridge are longtime rivals, but in February, the two universities launched their first-ever joint fundraising campaign in order to save the Lewis-Gibson Genizah Collection—named for the intrepid twins who led Schechter to it and, not incidentally, endowed Westminster College, which owns the collection but can no longer afford to keep it—from division and dispersal. (The New York-born, Oxford-educated financier Leonard Polonsky has already promised £500,000 of the £1.2 million needed.) This uncommon partnership is a testament to the value of the collection, which is the largest assembled from Ben Ezra. (The Jewish Theological Seminary of America, which Schechter would go on to lead as President, holds the second largest.) Ben Outhwaite, the head of genizah research at Cambridge, explained to me how crucial the Cairo Genizah collection is for scholars. “It is not hyperbole,” he wrote, “to talk about it as having rewritten what we knew of the Jews, the Middle East, and the Mediterranean in the Middle Ages.”

According to Jewish law, religious writings must be interred if they bear the name of God. The Jews of Fostat, though, preserved not only sacred texts but just about everything they ever wrote down. It’s not precisely clear why, but Outhwaite told me that medieval Jews hardly wrote anything at all—whether personal letters or shopping lists—without referring to God. (Addressing a man might involve blessing him with one of God’s names; an enemy might be cursed with an invocation of God’s malice.) David Kraemer, a professor of Talmud and Rabbinics at the Jewish Theological Seminary, explained that Fostat’s Jews spoke in Arabic but wrote in Hebrew—the Holy Tongue—and may have viewed the alphabet itself as sacred.

To this day, synagogues collect expired prayer books and ritual objects, and bury the contents every few years. Historians were doubly lucky with the worshippers at Ben Ezra, who not only deposited written texts into the genizah, but, for some reason, never buried its contents. (Instead, they stored it in what was literally a hole in the wall). As a result, we have a frozen postbox of some two hundred and fifty thousand fragments composing an unparalleled archive of life in Egypt from the ninth to the nineteenth centuries. The community may have been somewhat atypical—many of its Jews were wealthy, living at the center of a mercantile network, and Fostat was safer for Jews than the Land of Israel. Still, scholars can extrapolate a great amount of information from the Genizah documents about life for Jews during the Islamic Period in cities such as Baghdad, Damascus, and Aleppo. No other record as long or as full exists.

For centuries, historians had relied for their understanding of restrictions on Jewish life on Islamic legal documents mandating that Jews carry bells and wear badges and distinguishing clothing. But the contents of the Cairo Genizah show that Jews were allowed a far more vibrant lifestyle, and treated much more tolerantly, than had been assumed. The Fatimid Caliphate, a dynasty that ruled from 909-1171, “embraced the organs of Jewish government even to the point of financially supporting the ancient Academy of Jerusalem, promoting self-governance by the Jewish community and assisting the progress of pilgrims to the holy sites,” Outhwaite said. Jewish merchants partnered with Christians and Muslims; they ran perfume shops and silk weaveries together. Hundreds of letters buried in the genizah show that Jewish merchant princes set sail from Egypt or Yemen to India and returned along the Red Sea and Malabar Coast if they didn’t marry Indian women and settle there. Marriage contracts in the collection show that divorce was common. While very few Jews married Christians and Muslims, there is ample evidence of close relationships with interfaith neighbors, like letters seeking rabbinical advice about husbands who kept apartments for their Muslim concubines. Instances of day-to-day anti-Semitism were less common than imagined.

The Cairo Genizah also preserved telling artifacts of Biblical and Hebrew literature, like a large leaf of the great twelfth-century scholar Moses Maimonides’s Commentary on the Mishnah in his own “very distinctive (i.e. messy) handwriting,” an exceptional tenth-century vellum copy of the Jewish sage Saadya Gaon’s translation of the Bible into Arabic, and an autographed poem by the Spanish Hebrew poet Joseph ibn Abitur. It contained Torahs and Talmuds from all over the world. Beyond these canonical works, the Genizah reveals profane and even occult texts related to superstition and magic; it holds spells for erotic conquest, and others for inflicting bodily harm. (One leaf had this enchantment to make a woman sleep with you: “Take your trousers and put them on over your head, so that you are naked. Say: ‘So-and-so son of So-and-so is doing this for So-and-so daughter of So-and-so, in order that she will dream that I sleep [with] her and she sleeps with me.’”)

But it is the social history of Fostat’s Jews that the Genizah colors in most splendidly. We see what people bought and ordered, and what got lost in shipments between Alexandria and the Italian ports. We learn what clothes they wore: silks and textiles for the middle classes, from all over the known world. The Genizah includes prenuptial agreements and marriage deeds from the eleventh century listing the full inventory of a woman’s trousseau. It also contains the oldest-known Jewish engagement deed, from 1119, which was invented to grant a woman (and her dowry) legal protection as the time period between betrothal and marriage changed in medieval Egypt.