[1][2] According to the Tanakh, King Hezekiah prepared Jerusalem for an impending siege by the Assyrians, by "blocking the source of the waters of the upper Gihon, and leading them straight down on the west to the City of David" (2 Chronicles 32:30).

An older water system, sometimes called the Siloam Channel, partly fulfilled a similar purpose and dates back to the Canaanites.

Since the Gihon Spring was already protected by a massive tower and was included in the city's defensive wall system, Jerusalem seems to have been supplied with enough water in case of siege even without this tunnel.

According to Aharon Horovitz, director of the Megalim Institute, the tunnel can be interpreted as an additional aqueduct designed for keeping the entire outflow of the spring inside the walled area, which included the downstream Pool of Siloam, with the specific purpose of withholding water from any besieging forces.

The curving tunnel is 583 yards (533 m; about 1⁄3 mile) long and by using the 12 inch (30 cm) altitude difference between its two ends, which corresponds to a 0.06 percent gradient, the engineers managed to convey the water from the spring to the pool.

In 1899, an ancient channel, also leading from the Gihon Spring halfway to the Siloam Pool area, but by a more direct route, was found.

Ronny Reich determined that it was constructed around 1800 BC, in the Middle Bronze Age, and thus that the spring's water had already been diverted many centuries before Hezekiah.

As originally constructed, it is understood as a 20 feet deep ditch in the ground, covered over by large rock slabs, which were then hidden in the foliage.

In addition to the (3 ft high) exit near the Siloam pool, the channel has several small outlets that watered the gardens facing the Kidron Valley.

(2 Kings 20:20)"When Hezekiah saw that Sennacherib had come and that he intended to wage war against Jerusalem, he consulted with his officials and military staff about blocking off the water from the springs outside the city, and they helped him.

(2 Chronicles 32:2–4)"It was Hezekiah who blocked the upper outlet of the Gihon spring and channeled the water down to the west side of the City of David.