Tuesday, August 5, 2008

Hezekiah’s Tunnel

Hezekiah’s Tunnel, an eighth-century-b.c. subterranean aqueduct in Jerusalem, was a magnificent engineering achievement. Teams of stonecutters, working no more than two abreast and using hand tools, cut the 1,730-foot (576-meter) passageway of bedrock, probably in about seven months. Starting from both ends, between 33 and 150 feet (10 and 45 meters) underground, without sophisticated surveying instruments or contact with the surface, they were able to reach a meeting point.

The Canaanite citadel called Jebus stood on a slope that fell away into a deep valley outside the present-day walls of Jerusalem’s Old City. It had a defensible water supply upon which the conquering Israelites were to build, reaching a climax in the reign of Hezekiah, King of Judah (727–698 b.c.). Jerusalem depended on a single source of water: the Gihon (or Gichon) Spring. Fed from underground streams and hidden in a small cave on the city’s eastern slope, it also irrigated surrounding farmland through canals built along the Kidron creek bed. Archeologists have found evidence of Canaanite fortifications designed to protect the spring. Gihon’s name describes its erratic nature: the Hebrew word means “eruption” or “gushing.” Although reliably producing up to 245,000 gallons (1.1 million liters) a day, the spring would flow profusely for half an hour, then reduce to a trickle for between four and ten hours—longer intervals in summer, shorter in winter.

A response to siege warfare generated the sophisticated water-reticulation systems that culminated in Hezekiah’s Tunnel, one of the great engineering achievements of ancient Jerusalem. Possibly as early as 1800 b.c., the Jebusites were able to reach Gihon from within their walls: a diagonal tunnel, like others in the region, followed a natural rock fissure to a point from which pitchers could be lowered to the spring. Some scholars believe that this was the passageway mentioned in the Bible, through which Joab led King David’s men into the city, which they then overthrew. It is known as Warren’s Shaft, for Colonel Charles Warren, an Englishman who discovered it in 1867. The debate continues over its date and who built it.

The Israelites augmented this basic system in two stages. First, they built the Siloam (Shiloah) Channel, probably during the peaceful reign of King Solomon (970–928 b.c.). From Gihon a part-open, part-tunneled conduit ran south along the Kidron brook to a reservoir in the HaGal (Tyropoeon Valley) at the southwestern corner of Jerusalem, which by then had been extended to what are now known as the Jewish and Armenian Quarters. Sluices along its eastern side had stone gates that could be opened to irrigate the gardens and fields in the valley below.

Ancient Jerusalem’s most extraordinary hydraulic engineering project—perhaps better classified as a civil defense undertaking—was Hezekiah’s Tunnel, discovered in 1838 by the American scholar Edward Robinson. Under the implacable Sennacherib (705–681 b.c.), the Assyrian Empire extended from the Persian Gulf to the Black Sea, and westward to the Nile valley. His father Sargon had overrun the northern kingdom of Israel, and Sennacherib was concerned with consolidating the family conquests. Hezekiah, the charismatic ruler of the relatively puny kingdom of Judah, reassured by the prophet Isaiah that God would protect Jerusalem, stood against the Assyrian might. He stockpiled weapons and extended the city’s defenses by building the 23-foot-thick (7-meter) Broad Wall. And at the first inkling of invasion he had devised a measure that would help his people survive a siege. He planned “to stop the water of the springs that were outside the city [and] closed the upper outlet of the waters of Gihon and directed them down to the west side of the City of David” (2 Chron. 32:30).

Hezekiah’s Tunnel (701 b.c.), still a functioning watercourse almost 3,000 years later, connects the Gihon Spring and the Pool of Siloam (or Hezekiah’s Pool), specially constructed at the south end of Jerusalem, where the king had extended the outer defenses. Thus the Bible calls the pool “the reservoir between the two walls” (Isa. 22). The direct distance between spring and reservoir is about 1,100 feet (330 meters), but the winding tunnel is 1,730 feet (576 meters) long. On average, it is about 3 feet (900 millimeters) wide and varies between 3 and 9 feet in height; in places, it is 150 feet (45 meters) beneath the surface of the hilly city. The fall from Gihon to Siloam is about 6 feet (1.8 meters), that is, a grade of about 1 in 70. The tunnel was excavated by two groups of workers, starting at each end and cutting toward each other through the rock to eventually connect. The Siloam Tunnel Inscription, engraved on one of the walls and found in 1880, celebrated their meeting:
While there were still three cubits to be cut through, [there was heard] the voice of a man calling to his fellow, for there was an overlap in the rock on the right [and on the left]. And when the tunnel was driven through, the quarrymen hewed … each man toward his fellow, axe against axe; and the water flowed from the spring toward the reservoir for 1,200 cubits, and the height of the rock above the head[s] of the quarrymen was 100 cubits.

Intriguingly, the indirect course of Hezekiah’s tunnel penetrated hard rock while missing softer deposits.
Several explanations have been offered for its meandering course. Perhaps the diggers followed a sequence of fissures and crevices that pock the limestone under Jerusalem; perhaps they tried to avoid disturbing the tombs of the kings; or perhaps there was a need to continually reorient themselves—evidenced by a number of false starts and the angle at which the two parts meet.

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