By Guy Gugliotta
Washington Post Staff Writer
Thursday, September 11, 2003; Page A03

In a rare success by scholars attempting to match existing archaeological structures with events in the Bible, researchers say they have shown conclusively that Jerusalem's King Hezekiah built the meandering Siloam Tunnel beneath the city around 700 B.C. to discourage the Assyrian Empire from laying siege.

The tunnel runs 1,749 feet southwest from Gihon Spring to the Siloam Pool beneath the ancient section of Jerusalem known as the City of David. Although it is knee-deep in water even today, it is no longer part of the municipal water system.

In research reported today in the journal Nature, geographer Amos Frumkin of the Hebrew University in Jerusalem and two colleagues used radiocarbon and other radioactive isotope testing to establish the age of the tunnel.

The new research confirmed what a majority of scholars had long believed -- that the tunnel was built about 700 B.C. and is almost certainly Hezekiah's "conduit," mentioned in II Kings 20:20 and further described in II Chronicles 32:3-4. "So there was gathered much people together, whom stopped all the fountains, and the brook that ran through the midst of the land, saying, Why should the kings of Assyria come, and find much water?"

"It is the first well-known biblical structure to be radiographically dated," Frumkin said in a telephone interview from Jerusalem. And together with a 100-word inscription that describes the construction and appears to come from Hezekiah's time, Frumkin said the weight of evidence should now effectively put an end to a small school of thought that held that the tunnel was built 500 years later.

"This was a preposterous suggestion that no one who knew anything about paleography ever accepted," said Harvard's Lawrence Stager, an expert in the archaeology of ancient Israel. The new research, he added, "basically reconfirmed the date that most archaeologists and epigraphers had suggested."

Left unanswered, however, are several questions that have intrigued scholars for years: How did two teams of excavators, working from opposite ends of the tunnel, manage to join up, and did their difficulty in finding each other account for the circuitous route the tunnel takes? How did the excavators ventilate the tunnel? There were no vertical shafts cut along its course, so the digging teams had to finish their work hundreds of yards deep into the rock.

"We hope to solve some of these puzzles," Frumkin said. "There are a lot of ideas out there. Maybe it meanders because the excavators believed they could not meet deep in the mountain, and went closer to the surface where they could hear the other team working."

Scholars for years thought that Hezekiah ordered the tunnel constructed to secure Jerusalem's water supply in anticipation of the arrival of King Sennacherib's Assyrian armies. Sennacherib, who spent most of his career putting down revolts by peoples conquered by his father, Sargon, besieged Jerusalem but never entered it.

Recent excavations have challenged this version of events. These show that Gihon Spring already lay within Jerusalem's battlements when Sennacherib laid siege, so "it's not so easy to know why the tunnel was built, since the water supply was already protected," Stager said. "Everybody figures it had something to do with the Assyrians, but they aren't quite sure what."

Stager suggested that Hezekiah might have wanted to build the tunnel so overflow spring water would flow to a reservoir within Jerusalem instead of passing through irrigation conduits into the Kidron Valley east of the city, where the Assyrians could easily get to it. He likened the Siloam Tunnel to a bathtub drain, with the irrigation channels closer to the top, like overflow spouts.

Frumkin said the tunnel is about 2 feet wide and between 4 and 16 feet tall. It moves up and down and drops as much as 98 feet below the city, with the marks of the iron chisels that cut through the soft limestone still readily visible. Tourists can visit the tunnel today.

Frumkin said his team found several layers of plaster on the bottom and sides of the tunnel, apparently used as a binder to prevent water from leaching away. The lowest layer covered the bare rock and contained a filler composed, among other things, of organic materials, charcoal and ash -- substances whose carbon content allows researchers to determine their age using radiographic analysis.

The team obtained its carbon dates from a piece of wood and plant material taken from the plaster. It then took cores from stalactites hanging from the ceiling, calculating the age of the deposit by measuring the presence of uranium and thorium isotopes. Taken together, the deposits suggested a construction date of 700 B.C.

Frumkin said the excavation "could have taken a few months or a few years -- we tried to do [an excavation] something like it." In either case, he added, he had no idea how the tunnel was ventilated during construction.

"The likeliest thing is that they had some sort of bellows at the tunnel portals," said civil engineer and tunneling expert Richard Robbins, an Edmonds, Wash.-based member of the National Academy of Engineering. The bellows would connect to a sleeve or tube that would extend into the tunnel all the way to the rock face.

Fresh air would be pumped in, and stale air would escape through the portal: "I have been 250 meters into small tunnels with no ventilation, but that was with flashlights and a short visit," Robbins said. "It must have been pretty awful for them."