Jerusalem, Israel, is one of the oldest continuously inhabited cities in the world, and a holy place for the Jewish, Christian and Muslim religions. The oldest part of Jerusalem, known as the City of David, owes its development to an important geologic feature: a freshwater source called the Gihon Spring. Originating at the base of the hill on which the City of David was constructed, the Gihon Spring’s waters emerge from a cave on the city’s eastern slope before flowing into the adjacent Kidron Valley. Professor Rehav Rubin, who teaches geography at the Hebrew University of Jerusalem, explains how the spring’s geography helped determine its name. “The term gihon’ in Hebrew is a derivative of ‘gushing’,” he says. “Due to geologic reasons, the inner cave where the water came from had the shape of a siphon. Therefore, the cave had to be full and then the water gushed out through this siphon tube. Then there was a burst, a gush of the water. Then after a large amount of water flowed out there was a break until the inner cave filled again.” Since Jerusalem is located in a region without many water sources, the spring proved instrumental to the city’s early development. “Basically, if you want to settle in this area, this is the water source,” Rubin says. “The Gihon. There is no alternative.” Though the Gihon Spring was what Rubin calls “the most important key factor” to the ancient city’s location, another geographic element also proved important. “The hill itself [on which the City of David was built] could be defended quite easily, because it is a pretty steep hill with valleys on both its eastern and western sides,” Rubin says. “There is desert . . . The only problem where you have to build something is on the north end edge.” Together, Rubin says, the hill and the spring guided Jerusalem’s emergence as an urban center. “You have water,” he says. “You have land. And, in case of emergency, you have the possibility to defend yourself.” Waterworks The Gihon Spring is located outside the ancient city’s walls. Early residents had to build tunnels to access their water supply when the city was under siege. The earliest of these waterworks is Warren’s Shaft—named after British engineer Sir Charles Warren, who discovered the subterranean water system in 1867. The entrance to Warren’s Shaft is found within the walled portion of the City of David. It is a rock-cut tunnel 41 meters (135 feet) long and has a vertical shaft at its eastern end. The shaft descends 13 meters (42 feet) into the waters of the Gihon Spring. Water could be drawn using a container on a rope. This made it possible for residents to attain water without having to leave the protection of the city walls. The latest—and most impressive—of ancient Jerusalem’s waterworks is probably the so-called Hezekiah’s Tunnel. The construction of the subterranean feature is described in the book of Chronicles in the Bible: “[Hezekiah] blocked up the upper spring of Gihon and brought the water down through a tunnel to the west side of the City of David.” “Hezekiah’s Tunnel was built by King Hezekiah before 701 BCE, when it helped Jerusalem to survive the siege by King Sennacherib of Assyria,” Rubin says. “It is a tunnel cut in the rock beneath the City of David leading water from the Gihon to the Siloam Pool [a freshwater reservoir fed by the tunnel].” The S-shaped tunnel is 533 meters (1,749 feet) long and winds beneath the City of David to connect the spring with a less-vulnerable area of the city. Scientists are not sure why the tunnel curves instead of traveling in a straight line. Some think that workmen might have cut the tunnel to follow naturally curving rock formations, while another theory suggests that ancient engineers might have simply worked to expand a pre-existing crack they discovered into the tunnel. The Gihon Spring was used by Jerusalem residents throughout the 20th century. “In antiquity, as far as I know, it was always reliable,” Rubin says. “However, as the city grew larger and larger they had to build other water reserves with large water systems to the houses.” Jerusalem currently gets its water from the area around Rosh Ha-Ain, about 40 kilometers (25 miles) northwest of the city. Even though the Gihon Spring is no longer vital to the survival of the city, the spring’s subterranean water tunnels are a popular tourist attraction in Jerusalem. “Today, tourists are going to the springs and to Warren’s Shaft and Hezekiah’s Tunnel every day,” Rubin says. “It’s a wonderful experience in the summer.”