Black Frost
The so-called Four Seas is actually one sea, but people often call what they see in the south the South Sea and what they see in the north the North Sea. This can be fully confirmed in historical records.
Yan Daofu(嚴道甫), while traveling in the areas of Shaanxi and Gansu, once met with Duke Wu(伍公), the sincere and resolute. Duke Wu told Yan Daofu: During the reign of Emperor Yongzheng, he was ordered to visit the area of Eluosi and heard that there was a sea at its northern border.