Dangun is the mythical founder of Korea. The oldest existing record of the foundation myth involving him appears in the Samguk Yusa, a 13th-century collection of legends and stories.
Dangun's ancestry begins with his grandfather Hwanin (환인; 桓因;), the "Lord of Heaven" (a name which also appears in Indian Buddhist texts). Hwanin had a son Hwanung who yearned to live on the earth among the valleys and the mountains. Hwanin chose Mount Taebaek (태백산; 太伯山) for his son to settle down in and sent him with 3,000 helpers to rule the earth and provide humans with great happiness. Hwanung descended to Mount Taebaek and founded a city, which he named Sinsi (신시; 神市), or "City of God." Along with his ministers of clouds, rain, and wind, he instituted laws and moral codes and taught the humans various arts, medicine, and agriculture.
A tiger and a bear living in a cave together prayed to Hwanung to become human. Upon hearing their prayers, Hwanung called them to him and gave them 20 cloves of garlic and a bundle of mugwort. He then ordered them to only eat this sacred food and remain out of the sunlight for 100 days. The tiger shortly gave up and left the cave. However, the bear remained and after 21 days was transformed into a woman.
The bear-woman (Ungnyeo; 웅녀; 熊女) was very grateful and made offerings to Hwanung. She lacked a husband, however, and soon became sad and prayed beneath a sandalwood tree to be blessed with a child. Hwanung, moved by her prayers, took her for his wife and soon she gave birth to a son, who was named Dangun Wanggeom (단군 왕검; 檀君王儉).
Dangun ascended to the throne in the 50th year of the reign of the Emperor Yao (a legendary Chinese sage Yao), the year of Gengyin, built the walled city of P'yŏngyang, and called the kingdom Joseon. He then moved his capital to Asadal on Mount Baegak (or Mount Gunghol). 1,500 years later, in the year Kimyo, King Wu of the Zhou Dynasty enfeoffed Jizi to Joseon, and Dangun moved his capital to Jangdangyeong. Finally, he returned to Asadal and became a mountain god at the age of 1,908.
It is often said that Dangun ascended to the throne in 2333 BC, based on the description of the Dongguk Tonggam (1485), but the date differs among historical sources; nevertheless, all of them put it during Yao's reign (traditional dates: 2357 BC-2256 BC). The Samguk Yusa say Dangun ascended to the throne in the 50th year of Yao's reign, while Sejong Sillok says the first year and Dongguk Tonggam says the 25th year.
Origin of the myth
Dangun has never appeared in Chinese documents, even though they record other legends like that of Jumong (the legendary founder of Goguryeo) in detail. In addition, the Samguk Sagi--the oldest existing history book in Korea--makes no mention of Dangun, leading some people to theorize that the myth was formed some time between the 10th and 13th centuries. But Kim Busik, the author of Samguk Sagi, was a pro-Sinicist, and it is possible he omitted records of pro-Three Kingdoms history. Later work, Samguk Yusa, which was meant to be a supplement of Samguk Sagi, describes more about Danguns.
Dangun and nationalism
While Shamanism, Buddhism, and Confucianism were the dominant religious and philosophical movements in Korea before the 20th century, Gosindo (고신도; 古神道) existed as a cult, which had largely died out by the 15th century. In the late 19th and early 20th centuries, with a resurgence in Korean nationalism, the movement was revised in the cults of Daejonggyo (대종교; 大宗敎) and Dangungyo (단군교; 檀君敎). The latter was promoted by Na Cheol (1864-1916), but could not survive the Japanese Colonial Period (1910-1945) (Taejonggyo (1999)/Tangun (http://www.adherents.com/adhloc/Wh_179.html)), since it conflicted with the state Shinto myth of the Japanese emperor's divinity.
Until the end of World War II, Japan promoted the myth that its emperors were desenceded from the goddess Amaterasu. On November 9, 1928--the eve of the deification of Hirohito and the official beginning of his imperial reign--the Donga Ilbo (a widely read Korean daily newspaper) snubbed the official Japanese celebrations in Korea and attempted to reinvigorate Korean nationalism by publishing an account of the Dangun myth (Bix 2001). After independence in 1945, Daejonggyo and Dangungyo were revived, but lacked mass appeal. Nevertheless, campaigns to teach the Dangun myth as historical fact in schools partially succeeded. Since 1988, the national history textbook has explained that Dangun Wanggeom was the ruler's title and that the legend of Dangun reflects historical fact, a move that has since been one-upped in Japan by attempts in 2000 to treat the mythical emperor Jimmu's reign as historical fact in a new textbook  (http://wwwsoc.nii.ac.jp/rekiken/english/appeals/appeal_001205.html).
Until 1961, the official South Korean era (for numbering years) was called the Dangi (단기; 檀紀), which began with 2333 BC, the year of the mythical founding of Joseon by Dangun. Daejong-gyo designated October 3rd in the lunar calendar as Gaecheonjeol (개천절; 開天節), or the "Festival of the Opening of Heaven." This day is now a national holiday in the Gregorian calendar, called National Foundation Day. This is also similar to the situation in Japan, where Jimmu's mythical foundation of that country is celebrated on February 11 of each year  (http://www.asij.ac.jp/elementary/japan/jp_holi.html#feb11).
In 1990s, in a move to try to legitimize itself, the North Korean government claimed it had found and excavated the Mausoleum of Dangun.
2,300 year ago: Koreans (Yayoi people) moved to Japan (by Japanese History based on DNA Research).
300 A.D.: YAYOI PEOPLE FROM KOREA migrated to Japan (by Y Chromosome DNA). The both of Yayoi and Jomon people found to be fathers to modern Japanese people.