Deciphering ancient religious scrolls is a project that is never really finished, but the results can be especially fascinating when one looks beyond the text—like the researchers who recently studied and interpreted some mysterious purple spots on a secret Vatican manuscript.

As Live Science is reporting, Italian researchers think they have finally solved the mystery of the strange purple spots that appear on animal skin parchment from a wide range of times and places. By studying an 800-year-old goatskin manuscript from the Vatican Secret Archive (which is much more well-known and studied than it sounds) that had been afflicted by the stains, the scientists were able to determine that they had been left by microscopic marine bacteria. The solving of one mystery, however, led to another: The document hadn’t been anywhere near the sea. “When my students came to me, saying, ‘Luciana, we found marine bacteria,’ I told them, ‘Repeat, please; there is a mistake. There must be a mistake!’” Luciana Migliore, the toxicologist with the University of Rome Tor Vergata who was behind the discovery, told Live Science.

The scroll preserves the story of a young soldier named Laurentius Loricatus, who accidentally killed a man and spent the rest of his life violently punishing himself in ritual penance for the sin. The document is a petition for his sainthood, created by his fellow villagers. The strange purple spots are found all over the parchment and nearly completely obscure the last section. Using bits of the parchment that had flaked off on their own, the researchers were able to identify the salt-loving, ocean-going bacteria, which were likely introduced when the skin was first washed (in seawater) to help preserve it, before it had ever even been written on. As the marine bacteria thrived over the years and centuries, they left behind the purple pigment.

Eventually the marine bacteria died off, and other bacteria came to live in the newly promising conditions they left behind, causing the purple spots to flake off and damaging the parchment. Hopefully, this revelation will help conservators repair similar parchments affected by the no-longer-mysterious purple blight.