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, according to an Associated Press report.

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 say. Excavation director Katie Demakopoulou says the find confirms that Midea, 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 gives us great joy,” she says. The passage would allow the people of Midea safe access to drinkable water even in times of enemy attack.

Excavations this past summer revealed rock-cut steps leading to the triangular passage, the entrance of which was covered with a large stone lintel. Up to 10 feet of its course are visible. “It advances under the walls, which are up to 18 feet thick, and probably led out of the citadel to a point where there was either an underground spring or well, or where water was brought from a distance through pipes,” Demakopoulou explains.

Controlling a strategic road in the northeastern Peloponnese, Midea was first occupied in the later Neolithic period, in the 5th millennium B.C. It flourished during Mycenaean times, and was destroyed by earthquake and fire at the end of the 13th century B.C. – after which the site diminished in size and significance. Traces of habitation also have been located from the Archaic (7th and 6th centuries B.C.), Roman and Byzantine periods.