Wayne Stiles has an excellent article this morning on Hezekiah's Tunnel. As Assyria began invading parts of the Kingdom of Judah, King Hezekiah decided to dig a tunnel from the Gihon Spring (located on the eastern side of Jerusalem near the Kidron Valley) to the western side. This allowed for the water to flow from the spring and make it accessible to the citizens of the city.
After these deeds of faithfulness, Sennacherib king of Assyria came and entered Judah; he encamped against the fortified cities, thinking to win them over to himself. And when Hezekiah saw that Sennacherib had come, and that his purpose was to make war against Jerusalem, he consulted with his leaders and commanders to stop the water from the springs which were outside the city; and they helped him. Thus many people gathered together who stopped all the springs and the brook that ran through the land, saying, “Why should the kings of Assyria come and find much water?”...This same Hezekiah also stopped the water outlet of Upper Gihon, and brought the water by tunnel to the west side of the City of David. Hezekiah prospered in all his works. - 2 Chronicles 32:1-4, 30
Even today, water still flows through this tunnel from the Gihon Spring to the Pool of Siloam. Tourists, if they don't mind getting a little wet, can walk the length of this tunnel.
Interior photograph of Hezekiah's Tunnel is courtesy of BiblePlaces.com.