The primary source of water in the City of David (Jerusalem) was the Gihon Spring, which surfaces in the Kidron Valley at the foot of the eastern slope. The Gihon Spring is an intermittent spring, whose waters gush forth several times a day and flow into the Kidron Valley. In order to exploit these waters, it is necessary to capture them in pools while the spring is flowing. Such pools were apparently built along the Kidron Valley and at the issue of the Central Valley in the southwestern part of the City of David. This is the lowest terrain in the vicinity of the spur, where the topographical layout makes it easy to construct reservoirs, channel the water into them, and efficiently defend the system.
The Siloam Tunnel was built to solve a specific hydrogeological problem of the Gihon Spring. It carried the water for a distance of some 400 m along the foot of the western bank of the Kidron Valley; part of it was an open channel and the other part was cut in the rock as a tunnel. Windowlike openings pierced in the eastern side of the channel permitted the diversion of water to irrigate plots of cultivated land in the valley. The Shiloh expedition reexcavated and cleared the Siloam Tunnel for a distance of approximately 120 meters. The tunnel carried the Gihon waters to the vicinity of the Siloam Pool, where they could be stored and their use regulated.
Rainwater runoff on the hill's exposed rock drained into the channel through openings in its roof. The main disadvantage of this system was that it was entirely outside the city's defenses and highly vulnerable in times of war and siege. This was the motive for cutting Hezekiah's Tunnel, which is essentially an aqueduct that winds through the Cenomanian rock. Its entire length (c. 533 m from the spring to the reservoirs at the issue of the Central Valley) is enclosed. The levels of the tunnel floor were carefully calculated: the height differential between the point of issue in the spring and the end of the tunnel is at most 35 cm, a very moderate gradient. The cutting of the tunnel was described by the workers in the famous Siloam Inscription, which was incised in the tunnel wall near its southern end. The tunnel is mentioned several times in the Bible in connection with Hezekiah's construction projects in Jerusalem (2 Kgs 20:20; 2 Chr 32:3–4, 30), which are now confirmed by the abundant archaeological evidence.
The inscription was accidentally discovered in 1880, about 6 m from the the tunnel's outlet to the Siloam Pool. This text of six lines was inscribed in beautiful letters in straight rows on the lower half of a rectangular area in the rock that had been smoothed beforehand. Perhaps the smoothed area was prepared to receive a longer text, or else the mason was unsure of just how much space the whole text would take.
The inscription was extracted from the rock surface by a Greek resident of Jerusalem who sought to sell it. In the processs the inscription was broken into six or seven pieces. The text was repaired on the basis of a squeeze that had been made by the Swiss architect Conrad Schick shortly after the initial discovery. The repaired stone was transferred to Istanbul where it is now on display in the Museum of Ancient Near Eastern Antiquities.
