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.

Dating 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.

The passage would allow the people of Midea, about 93 miles south of Athens, safe access to drinkable water even in times of enemy attack.

``It is a very important discovery, which gave us great joy,'' excavation director Katie Demakopoulou said.

Only three such networks - major engineering feats requiring intensive labor - from Mycenaean times have been found so far.

Excavations in late June and July at Midea revealed cut rock steps leading to the triangular passage, whose entrance was covered with a large stone lintel. At the entrance to the 5-foot-high passage, archaeologists found quantities of broken clay water jars and cups.

The 6-acre site was girdled with a wall of huge stone blocks, built around 1250 B.C. Excavations have also uncovered several buildings - some decorated with painted plaster walls - pottery, a clay figure of a goddess, seal-stones and an amethyst vase shaped like a triton shell.

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 have also been located from the Archaic (7th and 6th centuries B.C.), Roman and Byzantine periods.

Greek state archaeologists and archaeologists from the Swedish Institute at Athens, a private foundation financed by the Swedish government, have systematically excavated Midea since 1983.