Deep in the archives of France’s national library, an assortment of coded letters listed as Italian texts lay untouched for more than 400 years. But when three code breakers — a German pianist, an Israeli computer scientist and a Japanese physicist — stumbled upon them, they discovered something remarkable.

They were, they found, not Italian texts at all.

Instead, they were part of the secret prison correspondence of Mary, Queen of Scots, whose tragic life and tangled role in the lethal dynastic and religious politics of 16th-century Europe have long fascinated writers and historians. One leading biographer of Mary described the discovery as the most significant in the study of her life for more than a century.

“We found treasure lying in plain sight,” said George Lasry, the Israeli computer scientist who led the yearlong project, which was released to the public on Wednesday, the 436th anniversary of Mary’s death.

Mary became the queen of Scotland when she was just 6 days old, in 1542, but was imprisoned and forced to give up her throne in 1567. She escaped to England, only to be jailed again by her cousin Queen Elizabeth I as a threat to her own rule. After 19 years as a prisoner, she was eventually executed in 1587, at age 44, accused of involvement in a Catholic plot to assassinate the Protestant Elizabeth.

The 57 letters, written between 1578 and 1584 and previously believed lost, include her thoughts about her ailing health, her conditions as a captive in a series of English castles and her failed attempts to secure her freedom.