This story appears in the March 2017 issue of National Geographic magazine.

Built in the sixth century at the foot of Mount Sinai in Egypt, St. Catherine’s Monastery is the world’s oldest such institution in continuous use. Its library preserves hundreds of manuscripts collected during medieval times—classical texts, scriptures, and other documents of interest to the monks. But it turns out that people recycled the pages of some of those manuscripts, erasing texts they no longer needed. Since 2011 the monastery has been working to recover some of those long-lost erasures using modern digital technology.

About half of the library’s manuscripts were written on parchment, the specially prepared skin of a calf, goat, or sheep. Parchment can be recycled by scraping off any ink and writing on the fresh surface. The old text isn’t entirely gone, though. It remains embedded in the page as a ghostly shadow, which can be resurrected with a technique called multispectral imaging, designed to peer into both visible and invisible wavelengths of light.

So far the imaging has revealed some 6,800 hidden pages in 74 of the monastery’s 163 recycled parchments, called palimpsests. “We have identified erased texts in 10 languages that date from the fifth to the 12th centuries,” says Michael Phelps, the director of the recovery effort. In the example above, a text in Syriac overlays a ninth-century translation of a page from a medical treatise by the ancient Greco-Roman physician known as Galen.