The only natural fresh water source around Jerusalem is the Gihon spring located just to the east of the city. Originally, there was an aqueduct that brought water into the city, but it was at ground level and could relatively easily be attacked by opposing armies, as the spring was located outside the city walls due to engineering concerns. King Hezekiah in anticipation of the Assyrian armies' arrival dug an underground tunnel/aqueduct that went straight into the city. This tunnel, known now as Hezekiah's Tunnel, was rediscovered in 1838 and has since been open to the public to walk through. For more details see here as well as numerous photos and information available through Google.
Sources: I've walked through it!