King Hezekiah had a tunnel constructed which carried water from the Gihon Spring outside the city to the Pool of Siloam inside the city of Jerusalem.
"When Hezekiah saw that Sennacherib had come, intent on making war against Jerusalem, he consulted with his officers and warriors about stopping the flow of the springs outside the city, and they supported him. A large force was assembled to stop up all the springs and the wadi that flowed through the land, for otherwise, they thought, the king of Assyria would come and find water in abundance" (2 Chronicles 32:2-4).
These events occurred in the year 701 BCE, when the Assyrian king laid siege to Jerusalem. The Gihon Spring, which was outside the city, confronted King Hezekiah with a double dilemma: to ensure water for the besieged city, yet to deny the source of the water to the Assyrian forces. The Bible describes Hezekiah's solution: "It was Hezekiah who stopped up the spring of water of Upper Gihon, leading it downward west of the City of David "(2 Chronicles 32:30). The waters of the Gihon were diverted into the Gai wadi by means of a tunnel 533 meters (581 yards) long, which was hewed from both ends simultaneously, probably along the course of a natural cleft in the rock. An inscription in the rock at the end of the tunnel describes the completion of the project.