Hercules, Pillars of, a name given by the ancient Greeks to a pair of rocks at the Strait of Gibraltar, the entrance to the Mediterranean Sea. The northern, or European, rock is Gibraltar (ancient name: Calpe). The southern rock is thought to be either Jebel Musa or Mount Acho (ancient name: Abyla). According to one legend, the pillars were part of a mountain range until Hercules tore them asunder.