Men walk on the narrow paths between vats of lye and colorful dyes in a leather tannery in Fez, Morocco. The leather tannery, located in the Medina of Fez, an UNESCO World Heritage site, features leather-making techniques that remain unchanged since the Middle Ages. The Medina of Fez is an ancient and walled quarter of the city of Fez—one of the best-preserved old cities of the Arab world. The sprawling Medina of Fez is also the world’s largest car-free urban zone.