It’s out in the open, but easy to overlook the mystery within Piazza Vittorio in Rome’s Esquilino neighborhood. The Porta Magica (“Magic Door,” a.k.a. “Alchemist’s Door”) is the only remaining entrance to the 17th-century villa of Massimiliano Palombara, who became obsessed with a recipe to turn metal into gold. Given to him by an alchemist, the recipe was written in a secret code that Palombara was unable to read. At a loss, he had the recipe inscribed into the door of his home, hoping that one of The Alchemists of Palazzo Riario—a local alchemy group—would pass by and recognize the language. It still stands, just waiting for someone to crack the secret formula.