(…which I’m told are forever)

滄海変じて桑田となる
(Soukai henjite souden to naru; “The sea becomes a field of mulberries.”)

Definition:

The world is full of dramatic changes. If you stop paying attention to something for a few years, when you next notice it, it will be as different as if land had risen out of the depths of the sea and been cultivated as a mulberry orchard.

Breakdown:

We begin with the noun 滄海 (soukai), “ocean.” Any particles have been elided. Next is the intransitive verb 変じる (henjiru), “to change (into),” to transform,” in conjunctive form. Following this, we find the noun 桑田 (souden), “mulberry field,” marked by the particle と (to) as the result of the verb なる (naru), “to become,” which appears in conclusive form.

Notes:

桑田 is also a family name, commonly read as Kuwata or Kuwada. Note, however, that the correct pronunciation for this phrase allows a play on words with both the beginning and end states using words that start with sou.

This kotowaza reportedly comes to us from the Shenxian Zhuan (『神仙伝』, in Japanese Shinsenden), a 4th century CE Daoist text purporting to be contain the biographies of various supernatural beings. Perhaps as a result of its antiquity, the saying can take a number of forms, including four-character compound 滄海桑田 (soukai souden), contracted noun phrase 滄海の変 (soukai no hen), or simple reversal 桑田変じて滄海となる (souden henjite soukai to naru).

Note that mulberry trees themselves take about a decade to grow from seed to fruiting maturity, so the depicted change is more dramatic than if the sea-bed had simply become a grassy field.

Example sentence:

「滄海変じて桑田となる、いつの間にかスーパーコンピュータが手のひらに入るほど小さくなってしまいました」

(“Soukai henjite souden to naru, itsu no ma ni ka suupaakonpyuuta ga te no hira ni hairu hodo chiisaku natte shimaimashita.”)

[“The world is always changing; at some point supercomputers became small enough that they could fit in the palm of your hand.”]