Kim and I were able to experience the Holy Land in May 2018. We stayed in Jerusalem for about four days, and on our last day, a small group of us who took off into Jerusalem for a couple of hours to walk through Hezekiah’s Tunnel, from the Gihon Spring to the Pool of Siloam. I found this video that helps you see what it was like.
Hezekiah’s Tunnel was cut through bedrock in 701 BC under the City of David, curving and weaving for 1750 feet. If the same tunnel were cut in a straight line, it would be 40% shorter at only 1070 feet. This tunnel was cut to bring water from the Gihon Springs in the Kidron Valley (on the east outside the city’s walls) through the bedrock and into Hezekiah’s city of Jerusalem (at the Pool of Siloam).
After all that Hezekiah had so faithfully done, Sennacherib king of Assyria came and invaded Judah. He laid siege to the fortified cities, thinking to conquer them for himself. When Hezekiah saw that Sennacherib had come and that he intended to make war on Jerusalem, he consulted with his officials and military staff about blocking off the water from the springs outside the city, and they helped him. 
A large force of men assembled, and they blocked all the springs and the stream that flowed through the land. ‘Why should the kings of Assyria come and find plenty of water?’ they said. Then he worked hard repairing all the broken sections of the wall and building towers on it. He built another wall outside that one and reinforced the supporting terraces (Millo) of the City of David. He also made large numbers of weapons and shields. . . 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. He succeeded in everything he undertook. – 2 Chronicles 32:1-5, 30
As for the other events of Hezekiah’s reign, all his achievements and how he made the pool and the tunnel by which he brought water into the city, are they not written in the book of the annals of the kings of Judah? – 2 Kings 20:20