In China, at least since the middle of Tang dynasty, the phrase mulberry fields (Hanzi=桑田; pinyin=sāngtián; Japanese=souden, Vietnamese=tang điền) is a metonymy for the land which was or will be covered by oceans. This term is often used in Chinese literature and poetry, for example in Zuo Zhuan, which is about the death of Duke Jing of Jin and mentions the "Shaman of Mulberry Fields" (Chinese: 桑田巫; pinyin: sāngtián wū). Along with the "blue seas" phrase, since the Han dynasty, these two phrases were combined into an idiom that has meaning about changes and changing.