The sea and mulberry fields
Refers to large-scale transformations or changes (of places, people, etc)
Neutral
那个城市虽然历经沧海桑田依然健在 | Nàgè chéngshì suīrán lìjīng cānghǎisāngtián yīrán jiànzài
The city still stands though much has changed
[Source]
那老人说: “30年前这里还很荒芜,但今天高楼林立,真是沧海桑田般的巨变” | Nà lǎorén shuō: "30 Nián qián zhèlǐ hái hěn huāngwú， dàn jīntiān gāolóu línlì， zhēnshi cānghǎisāngtián bān de jùbiàn."
That old person says, "Thirty years ago, this place was overgrown with weeds, but now it's filled with skyscrapers. The transformation has truly been incredible."
[Source]
None
桑田沧海 (sāng tiān cāng hǎi)
The original text of this story doesn't offer an explanation as to who 蔡经 (Cài Jīng) is and why he's relevant in this story. I did some digging, and it looks like he was a mortal man who achieved immortality through enlightenment.
蓬莱仙岛 (péng lái xiān dǎo) is a mythical mountain where immortals often reside. You can find out more on Wikipedia
《神仙传》(Shén Xiān Zhuàn), a collection of ten scrolls by the Jìn Dynasty (晋, 265 - 420) Daoist 葛洪 (Gě Hóng). The scrolls tell the stories and deeds of 92 celestial beings.
In the Later Hàn Dynasty (西汉, 206 BC - 8 AD), the immortal being 王方平 (Wáng Fāng Píng) descended to the mortal world to visit 蔡经 (Cài Jīng). After a while, the immortal 麻姑 (Má Gū) also arrived. 麻姑 looked to be around 18 or 19 years old, and she was very beautiful. Some of her hair was coiled in a bun atop her head, while the rest fell to her waist. Her clothing was made out of an unknown fabric, but it had multi-colored patterns embroidered into it and was absolutely resplendent.
She entered 蔡经's house and went to greet 王方平, who stood and welcomed her warmly. They all then sat so they could eat, and 麻姑 ordered that the food she had brought with her be served. The following meal was extremely luxurious. The cups they used were made out of jade, the plates made out of gold, and the food consisted of rare plants, fruits, and even meat from the mythical qílín (麒麟). Everything smelled wonderful too.
As they ate, 麻姑 said to 王方平, "Since I became an immortal, I have seen the sea dry and turn into mulberry fields three times. I have just been to 蓬莱仙岛 (péng lái xiān dǎo), where I noticed that the water has become shallow again. I wonder if the sea will turn to land once more."
Hearing this, 王方平 laughed and said, "The sages all say that the sea will indeed dry up and start kicking up dust."