沧cāng海hǎi桑sāng田tián

Literal: blue sea, mulberry fields
Figurative: Change so profound over a long span of time that the old world is no longer recognizable; usually said wistfully.

Where does 沧海桑田 come from?

It comes from the 《神仙传》 by Ge Hong, of the Jin dynasty, a collection of lives of immortals. In the tale of the immortal Magu, she tells another immortal, Wang Fangping, that in the time that has passed she has seen the Eastern Sea turn into mulberry fields three times, and that the water around Penglai has shrunk by half, as if it were about to become land again. The image of a sea turning into farmland, and more than once, conveys a span of time far beyond a human life. Today it is used for the great changes the years bring, often wistfully.

Source (出处): 葛洪《神仙传》

How is 沧海桑田 used? Two examples

回huí到dào小xiǎo时shí候hou住zhù的de村cūn子zi，才cái发fā现xiàn早zǎo已yǐ沧cāng海hǎi桑sāng田tián，老lǎo房fáng子zi一yī间jiān都dōu不bù剩shèng了le。
When I went back to the village where I grew up, I found it had long since changed beyond recognition; not one of the old houses is left.
几jǐ十shí年nián没méi见jiàn，当dāng年nián的de小xiǎo伙huǒ子zi都dōu成chéng了le白bái发fà老lǎo人rén，真zhēn是shì沧cāng海hǎi桑sāng田tián。
After decades apart, the young lads of those days had all become white-haired old men; the years had changed everything.

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