Chinese Traditional Story: Where did Xu Fu go?

In 219 B.C., the 28th year of Emperor Qin Shi Huang's reign, Xu Fu wrote a letter to Emperor Qin, saying that there are three sacred mountains in the sea, named Penglai, Fangzhang and Yingzhou where Immortals live. It was said that immortality herbs could be found there. So, Qin Shi Huang assigned Xu Fu to take 3,000 boys and girls on ships to search for it. How did this immortal herb come about? It is said that in Dayuan Kingdom in the Western Regions at that time, many people accidentally died, and their bodies were left on the road.