Mt. Penglai 蓬萊 was according to Chinese mythology located on an imaginary island in the Eastern Sea (Donghai 東海 or Bohai 渤海). It was one of the three (or five) mythical islands where immortals lived.
The book Shizhouji 十洲記 says that only immortals were able to reach this island. Nevertheless, the First Emperor of Qin 秦始皇帝 (r. 246-210 BCE) sent out an expedition to these islands with the attempt to obtain the herb of immortality that was said to grow there. He was instigated by a person called Xu Fu 徐巿, who submitted a memorial to the throne describing the three islands of Penglai 蓬萊, Fangzhang 方丈 and Yingzhou 瀛洲.
The treatise on the fengshan sacrifices (Fengshan shu 封禪書) in the history book Shiji 史記 also says that other regional rulers (zhuhou 諸侯) in earlier times attempted to reach these islands. All living creatures on these islands were said to be white, while the palace was of pure gold.
The book Liezi 列子 speaks of five islands that were called Penglai, Yingzhou, Fanghu 方壺, Yuanjiao 員嶠 and Daiyu 岱輿.
Today, a city on the northern shore of the Shandong Peninsula is called Penglai. Some Chinese nationalists try identifying the Penglai island with the Ryūkyū Islands (or even the island of Kyūshū), claiming that this is the reason why all islands in the Eastern Sea "belong to China".