曾经的沧海桑田
[céng jīng de cāng hăi sāng tián]
This phrase originates from a classical Chinese idiom describing vast changes over time—literal and figurative transformations as profound as oceans turning into farmland. Here, it refers to dramatic shifts experienced by the user in their own life or surroundings, possibly reflecting on the passage of time and personal growth or nostalgia for past times.