四海
The original Four Seas were a metaphor for the borders of pre-Han dynasty China. Only two of the Four Seas were tied to real locations, the East Sea with the East China Sea and the South Sea with the South China Sea. During the Han dynasty, wars with the Xiongnu brought them north to Lake Baikal. They recorded that the lake was a "huge sea" (hanhai) and designated it the mythical North Sea. They also encountered Qinghai Lake, which they called the West Sea, and the lakes Lop Nur and Bostang in Xinjiang. The Han dynasty expanded beyond the traditional West Sea and reached Lake Balkash, the westernmost boundary of the empire and the new West Sea of the dynasty. Expeditions were sent to explore the Persian Gulf, but went no further. The military expansion of the dynasty ended in 36 BC after the Battle of Zhizhi.