According to legend, two immortals—a man named Wang Yuan and a woman named Magu—once agreed to meet at Cai Jing's home for a drink.
On the appointed day, Wang Yuan arrived at Cai Jing's home in grand style, seated in a chariot pulled by five dragons, surrounded by a procession of attendants and musicians riding qilins. He wore a traveler's hat, a colorful sash, and a tiger-headed quiver, looking every bit the majestic immortal.
After Wang Yuan and his entourage descended into Cai Jing's courtyard, his attendants vanished in an instant. Wang Yuan then exchanged greetings with Cai's family and sat alone, awaiting Magu's arrival. Growing impatient, he waved toward the sky and ordered a messenger to summon her. None of Cai Jing's household knew which celestial maiden Magu was, so they could only crane their necks in anticipation.
After a moment, the messenger reported to Wang Yuan from midair, "Magua sends her regards and says it has been over five hundred years since she last saw you. She is currently inspecting the Penglai Immortal Island and will come to meet you shortly."
Wang Yuan nodded slightly and waited patiently. Before long, Magu descended from the sky, her attendants numbering only half of Wang Yuan's. Only then did the Cai family see that Magu appeared as a beautiful girl of eighteen or nineteen, with long hair reaching her waist and a dress of unknown material embroidered with dazzling patterns.
After exchanging greetings, Magu and Wang Yuan ordered the feast to begin. The tableware, all made of gold and jade, was exquisitely crafted, and the dishes were mostly rare flowers and exotic fruits, their fragrance intoxicating—a sight and taste the Cai family had never witnessed before.
During the feast, Magu said to Wang Yuan, "Since attaining the Way and receiving my heavenly mandate, I have personally witnessed the Eastern Sea turn into mulberry fields three times. Just now, when I arrived at Penglai, I saw the seawater half as shallow as before—could it be turning into land again?"
Wang Yuan sighed, "Indeed, the sages all say the waters of the sea are receding. Soon, dust will rise there once more."
After the feast concluded, Wang Yuan and Magu each summoned their celestial chariots and ascended to heaven.
Later, people summarized this myth as "the blue sea turned into mulberry fields," using it to describe the sea becoming farmland and farmland becoming the sea, symbolizing great upheavals in worldly affairs or the passage of vast ages.
Source: *Biographies of Immortals*, Chapter "Wang Yuan"
Meaning of the Idiom: Later, the Chinese idiom "沧海桑田" came to describe how the sea becomes farmland and farmland becomes the sea, symbolizing great upheavals in worldly affairs or the passage of vast ages.