3_Hezekiah_Tunnel_2 As Hezekiah prepared for Sennacherib’s approach at the end of the 8th century, he knew the city would need water supplies to survive the inevitable siege. II Kings 20:20 notes his solution, which is evident today in the engineering marvel called Hezekiah’s Tunnel (or, Siloam Tunnel). The tunnel is 533 m long with a 0.6% gradient from the mouth of the Gihon Spring to the Pool of Siloam, where it empties.