Downtown Beijing has several lakes called "hai" or sea. During the Yuan Dynasty, the Mongol rulers expanded them and built imperial palace of Dadu around them. Subsequent emperors of the Ming and Qing dynasties used the lakes as imperial gardens. Today, the three northernmost lakes, "Xihai" (West Sea), "Houhai" (Rear Sea) and "Qianhai" (Front Sea), collectively known as "Shichahai", are lined with bars and cafés and known for nightlife. To their south, "Beihai" (North Sea) is a city park, and "Zhonghai" (Central Sea) and "Nanhai" (South Sea) are part of the Zhongnanhai Compound, residence to China's leaders. This string of lakes used to form the main riverbed of the Yongding River, which now flows to the west. But 1,800 years ago, the Yongding flowed through Jishuitan and downtown Beijing and then into Longtan Lake and on to Tongzhou. A major flood in 295 A.D. devastated part of Beijing, then known as Ji. Just west of Wangfujing, there is still a stretch of Beijing called "Shatan" or Sandy Beach.