"We were looking for the road and suddenly we discovered it," Shukron said. "And the first thing we said was, 'Wow.'"

The walls of the tunnel - made of ashlar stones 3 feet deep - reach a height of 10 feet in some places and are covered by heavy stone slabs that were the road's paving stones, Shukron said. Several manholes are visible, and portions of the original plastering remain, he said.

Pottery shards, vessel fragments and coins from the end of the Second Temple period were also discovered inside the channel, attesting to its age, Reich said.

The discovery of the drainage channel was momentous in itself, a sign of how the city's rulers looked out for the welfare of their citizens by developing an infrastructure that drained the rainfall and prevented flooding, Reich said.

The discovery "shows you planning on a grand scale, unlike other cities in the ancient Near East," said Joe Zias, an expert in the Second Temple period who was not involved in the dig.

But what makes the channel doubly significant is its role as an escape hatch for Jews desperate to flee the conquering Romans, the dig's directors said.

The Second Temple was the center of Jewish worship during the second Jewish Commonwealth, which spanned the six centuries preceding the Roman conquest of Jerusalem. Its expansion was the most famous construction project of Herod, the Jewish proxy ruler of the Holy Land under imperial Roman occupation from 37 B.C.

As the temple was being destroyed by the Romans in 70 A.D., numerous people took shelter in the drainage channel and lived inside it until they fled Jerusalem through its southern end, the historian Josephus Flavius wrote in "The War of the Jews."

"It was a place where people hid and fled to from burning, destroyed Jerusalem," Shukron said. v Tens of thousands of people lived in Jerusalem at the time, but it is not clear how many used the channel to escape, he said.

About 100 yards of the channel have been uncovered so far. Reich estimates its total length will reach more than a half-mile, stretching north from the Shiloah Pool at Jerusalem's southern end to the disputed holy shrine known to Jews as Temple Mount and to Muslims as the Al Aqsa Mosque compound. The shrine is the site of the two biblical Jewish temples.