Hezekiah, king of Judah, fortified Jerusalem at the end of the 8th century BC, just before the invasion of Sennacherib. As part of his building project, Hezekiah brought water into the city of Jerusalem through a tunnel carved from over half a kilometer of bedrock (2 Kings 20:20). A six percent gradient was designed into the excavation to allow water to flow from the Gihon spring into the pool of Siloam (compare John 9:7).