35km/22mi north of Sousse, on a rocky stretch of coast interrupted by sandy coves, is the little town of Hergla (pop. 6,000), which is still relatively unspoiled by tourism. It occupies the site of ancient Horraca Caelia, which in the second century A.D. lay on the boundary between the two provinces of Byzacena and Zeugitania. It was completely destroyed during the Arab invasion. Hergla has a fine 18th century mosque, with a dome constructed in the "tube-vaulting" technique found at Bulla Regia. It was built over the tomb of a 10th century holy man named Sidi Bou Mendil, who according to his legend was able to turn his handkerchief into a flying carpet.