Seoul subway serving the Seoul Metropolitan Area is the longest subway system in the world. The total route length of the system extended as far as 940km as of 2013. The first line of the subway was opened in 1974 and the system presently incorporates 17 lines (excluding the Uijeongbu LRT and the recently opened Yongin Ever Line). The subway system is operated by multiple operators including the state-owned Seoul Metro, Seoul Metropolitan Rapid Transit Corporation, Korail, Incheon Transit Corporation, and other private rapid transit operators. Many extension projects are under construction on the already extensive subway network. The annual ridership of the subway system in 2012 was a mammoth 2.518 billion making it the second busiest subway system after Tokyo subway. Three Gorges Dam At 1.4 miles wide (2.3 kilometers) and 630 feet (192 meters) high, Three Gorges Dam is the largest hydroelectric dam in the world, according to International Water Power & Dam Construction magazine. Three Gorges impounds the Yangtze Riverabout 1,000 miles (1,610 km) west of Shanghai.