NARROWLY WINDING its way through more than 1,700 feet of Jerusalem limestone, the eighth-century B.C.E. tunnel of King Hezekiah is one of the great engineering feats of ancient times. The entire length of the tunnel was chiseled out by two teams digging toward each other from opposite ends of the City of David. Once completed, the channel carried precious water from the Gihon Spring just outside Jerusalem’s city walls to the more protected confines of the Siloam Pool. But working in near-total darkness with nothing more than small oil lamps and without the aid of intermediate surface shafts, how did the workers manage to find each other? And why did they follow such a circuitous route? Two prominent Israeli geologists think they have found the answer.