One of the least known, and most precious treasures in Cambridge is a hoard of ancient religious documents which scholars regard as equal in importance to the Dead Sea Scrolls.

In 1896 a Cambridge professor, Solomon Schechter, went to Cairo on the advice of two Scottish widows who, in their travel, had found a trove of Jewish documents and manuscripts in an old synagogue.

Jews, cautious about disposing of any written form of God’s name, have long been in the habit of taking old documents to the synagogue. Doing this over a millennium, worshippers at the Ben Ezra synagogue in Old Cairo inadvertently created a trove of sources for the study of Jewish social and religious history. Schechter took documents back to Cambridge by the crate-load — 140,000 fragments in all.

The fragments are known collectively as the Cairo Genizah (or Geniza) from the Hebrew for a document-store. Nearly a third of the materials are scattered around the world in universities and research institutes; the remaining two thirds are in Cambridge.

Now, documents in all locations are being scanned and catalogued and within five years should be available to the world via the web, thanks to an initiative launched in 1999 by the Friedberg Geniza Project, an international foundation. The main player in the project is the Taylor-Schechter Genizah Research Unit (widely called the Geniza Unit), set up in the mid-1970s to manage the Cambridge collection.

The importance to Jewish studies research of this online archive is expected to be revolutionary. The Geniza is “the greatest single hoard of primary sources for the study of Judaism and Jewish history ever uncovered,” says the University of Manchester professor Philip Alexander. The Dead Sea Scrolls, he argues, “have gained huge publicity because they are earlier in date and because they throw light on the origins of Christianity.” But the Cairo documents “are much larger in bulk and more varied in content, and they illuminate the ‘mainline’ rather than a sectarian ‘branchline’ of Judaism — Christianity.”

In the vault it is easy to see what Alexander is saying. Take, for example, Moses Maimonides, the 12th-century rabbi. Extraordinarily, there are original manuscripts of his in the collection — this is nothing less than a marvel for Jews, who conduct their daily rituals in accordance with his important legal writings, and study — and even sing in synagogue — his philosophy. Gallons of ink have been spilled over how Maimonides, the most renowned religious Jewish philosopher of all time, was influenced by his Muslim surroundings. Here, he tells you himself. A glance at one of his manuscripts shows something strange. “Hebrew letters are rounded and not joined up. Maimonides has slanted and in places joined up letters, seemingly emulating Arabic writing,” says Ben Outhwaite, head of the unit.

The Jerusalem Talmud, a text known to the world through various error-ridden versions, has become much better understood thanks to the earliest extant versions, found in the Geniza. Many were taken to Cairo by Jews who lived in the Holy Land but left because of the Crusades.

“People have been breaking their heads on passages in the Jerusalem Talmud for hundreds of years, having been unable to understand parts. Now they can say: ‘aah, this is what it should have said,’ now it makes sense,” says Stefan Reif, the unit’s senior academic consultant and former head. An independent Israel-based institute plans to publish a full Jerusalem Talmud incorporating all Geniza insights.

Many of the documents come from the period when Jewish culture was becoming less orally transmitted and more literary. So while until the 9th and 10th centuries prayers were learnt and recited by heart, then a prayer book began to take form. “Fragments from the Geniza give us remarkable insights in to how this prayer book, known as the Siddur and today used daily, began to come together,” says Professor Reif.

The discovery of the Geniza more than doubled the extant body of material written in Judeo-Arabic, the Arabic used by Jews in Arab lands and written in Hebrew characters.

Access to Arabic written in the letters of another language provides a rare insight in to how Arabic was pronounced 1,000 years ago. “Most Arabic texts of the period from other sources are written in a highly standardised literary language, that conceals the way people actually spoke,” says Geoffrey Khan, a Cambridge professor and expert in Arabic and other Semitic languages. “Many of the Arabic documents from the Geniza are written in a form of ‘sub-standard’ Arabic, which reflects more closely the real speech of the time.”

When the documents arrived in Cambridge, there were about 20 years of intense work. Then, as in other places that acquired material, a feeling set in that all the important discoveries had been made. However, in the 1950s, inspired by the trend towards sociological history, scholars began to explore the documentary material — information on every aspect of Jewish life. There are marriage contracts, business agreements, medical prescriptions, love poems and letters. This took off in earnest when Reif set up the unit to step up research, in the mid-1970s.

There is even an idea for women today who want to keep their husbands in check. Faiza, the son of Shelomo had such a reputation for going out drinking and hanging out with a bad crowd that Tuvia bat Eli made sure that her wedding contract — found in the Geniza — forbade such behaviour. Shelomo was also banned from taking slave girls for himself, unless Tuvia specifically permitted it. No record was found of whether the marriage lasted.

In the offices of the Geniza Unit, Esther-Miriam Wagner, a Hebrew and Arabic scholar, is looking at a small document — she is probably the first person to look at it in a millennium. She notes the main gist — it is a business letter — the size and language used, and types a summary for the digitising project.

When Reif took over the Geniza in the 1970s no such systemised approach was in place. Documents were still crammed in the tea chests originally shipped from Cairo.

The digitisation for web access now taking place represents the ultimate “democratising act”, according to Outhwaite. “What was once available only to those scholars with the research budgets is now becoming available to researchers worldwide or to anyone who is interested enough to take a look.”