Archaeologists excavating a sprawling prehistoric fortress in southern Greece have discovered a secret underground passage thought to have supplied the site with water in times of danger. Dated to the mid-13th century B.C., the stone passage passed under the massive walls of the Mycenaean citadel of Midea and probably led to a nearby water source, authorities said Friday. Excavation director Katie Demakopoulou said the find confirmed that Midea, about 150 kilometers (93 miles) south of Athens, had a sophisticated water supply system like those unearthed in the nearby citadels of Mycenae and Tiryns. "It is a very important discovery, which gave us great joy," she said. The passage would allow the people of Midea safe access to drinkable water even in times of enemy attack.