The emergency
Around 701 BC, Sennacherib of Assyria marched on Judah with the most feared army in the world. Jerusalem's water came from the Gihon Spring, which bubbled up outside the city wall, in the valley. Under siege, a city without water falls.
King Hezekiah made a plan:
"He took counsel with the princes, and the most valiant men, to stop up the heads of the springs, that were without the city... Lest the kings of the Assyrians should come, and find abundance of water."
2 Paralipomenon (2 Chronicles) 32:3 to 4
Hide the water from the enemy, and bring it inside for the city.
The Bible records it twice
"This same Ezechias was he that stopped the upper source of the waters of Gihon, and turned them away underneath toward the west of the city of David."
2 Paralipomenon (2 Chronicles) 32:30
And again: "how he made a pool, and a conduit, and brought waters into the city" (4 Kings 20:20).
Underneath. A conduit. Into the city.
The tunnel
It's still there. A tunnel cut through solid rock under the City of David, winding from the Gihon Spring to the pool of Siloam, about half a kilometre long. Water still runs through it, and visitors wade the whole length with a torch.
The inscription
In 1880, boys playing in the tunnel noticed writing carved into the rock wall. It was in ancient Hebrew, cut by the men who dug the tunnel.
It tells the story of the last day. Two crews had been digging towards each other through the rock, from opposite ends. When there were three cubits left to cut, the men on one side could hear the others:
"the voice of a man was heard calling to his fellow."
They dug on until they broke through and met. Then:
"Then the water flowed from the spring to the pool, a distance of one thousand and two hundred cubits."
The stone was cut out of the wall and taken to Istanbul, the capital of the Ottoman Empire, which ruled Jerusalem at the time. It's still in the Istanbul Archaeological Museum.
The siege
Sennacherib's army took the fortified cities of Judah and shut Hezekiah up in Jerusalem. Inside, the city had water. The Bible says God struck the Assyrian camp, and Sennacherib went home. Sennacherib's own record of the campaign boasts of everything else, and never claims Jerusalem.
Why this is a big deal
The Bible says Hezekiah turned the waters of Gihon underneath, into the city of David. Twenty-seven centuries later, they still flow that way, through the tunnel his men cut, past the words they carved on the day they met.