In 210 BCE, the first Emperor of China — Qin Shi Huang — sent an expedition to find the elixir of immortality. Leading it was a man named Xu Fu, a court sorcerer who claimed to know where the mythical islands of the immortals could be found. Xu Fu sailed east with a fleet of ships carrying thousands of young men and women, seeds, and craftspeople. He never returned.
What the Chinese records say
Chinese historical records note two voyages by Xu Fu. The first returned with excuses. The second, in 210 BCE, departed and was never heard from again. A later historian, Sima Qian, claimed that Xu Fu found a 'plain with broad swamps' where he declared himself king and simply stayed. No location is given.
The Japan connection
Japanese tradition has long associated Xu Fu with several locations, particularly around the Kii Peninsula and in parts of Kyushu. There are shrines dedicated to him. Local traditions claim descent from his followers. The timing is suggestive. The Yayoi people — who brought wet rice agriculture, bronze, and iron to the Japanese archipelago — arrived in Kyushu from the continent around the third century BCE. The Xu Fu expedition was in 210 BCE. These two facts don't prove anything, but they're not unconnected by time and direction.
What we can actually say
Mainstream archaeology does not accept the Xu Fu-as-founder-of-Japan hypothesis. What the Xu Fu story does tell us, reliably, is something about how ancient Chinese and Japanese cultures thought about each other — and about the universal human desire for a place beyond the known world where death cannot follow.
Whether he found it or not, Xu Fu pointed east. And east, people kept going.