The extent and peculiarities of the underground water system were discovered and explored in the 19th century. The passages were all connected to Gihon Spring, the Old City's sole source of fresh water and the reason that a city came to be built there. Modern Jerusalem's water supply is piped in from Lake Tiberias.

From Gihon Spring, which is in a cave, there runs a short, irregular tunnel leading to a vertical shaft that goes straight up 37 feet. This is called Warren's Shaft, after the British engineer Charles Warren, who explored it in 1867.

Someone standing on a rock platform at the top of the shaft could drop a bucket on a rope and draw up the cool water. A gently sloping tunnel, and then a steeper one, connect the platform with an entryway at the surface. Though the spring is a little outside the wall, the entryway to Warren's Shaft is safely inside.

Another important component, Hezekiah's Tunnel, was rediscovered in 1837 by Edward Robinson, an American Orientalist. The tunnel, drawing on the same spring, runs from the base of Warren's Shaft until it debouches in an open reservoir known as the Pool of Siloam. An inscription on the tunnel wall, written in ancient Hebrew script, tells how two teams digging from opposite ends managed to meet in the middle. That was an achievement that scholars found virtually inexplicable because of the winding route the tunnel followed, but the new findings show that the workers were actually following and widening the route of existing passages.

Systematic explorations were not renewed until 1978, when the late Dr. Yigal Shiloh, an Israeli archeologist, began research on the City of David. Dr. Gill, as the project's chief geologist, re-examined the waterworks and in 1980 began to recognize a clear example of function following form.

Beneath the City of David, he found, lie two layers of rock, highly porous limestone on top of more impervious dolomite. Warren's Shaft is a natural sinkhole that developed along a joint in the limestone. Its bottom narrows into a funnel-like shape, typical of a karstic sinkhole, and carbon dating of the calcium crust on its walls indicates an age of more than 40,000 years.

"This provides unequivocal evidence that the shaft could not have been dug by man," Dr. Gill wrote.

But the inhabitants of ancient Jerusalem probably took a hand to make the sinkhole into a well, he said, by widening the fissure between it and the spring and by sealing the bottom of the shaft to prevent leakage. Likewise, the people modified natural passages from the top of the shaft to the surface, widening them and cutting out steps.