Once upon a time, Hwan-ung, the Prince of Heaven, went to his father Hwan-in, the Ruler of Heaven, and asked him for the beautiful peninsula of Korea, so that he could govern it and live among the human beings.
So it came to pass that Hwan-ung and three thousand Heavenly followers landed in Korea under a sacred (well, it's sacred now) sandalwood tree on Mount Taebaek. Living near the tree were a tiger and a bear who yearned to become human beings. They prayed every day that they might become human. Hwan-ung heard their prayers and called them to him.
He gave them both a bundle of mugwort (also known as wormwood) and twenty garlic bulbs as their only food, and sent them to live in a cave for 100 days. "At the end of that time, you will become human," he said.
Well, the tiger couldn't stand it, being stuck in a cave eating garlic, so he soon gave up and ran off. But the bear kept at it. As the days passed, she noticed her hair was falling out, and lo! after only twenty-one days, she was transformed into a beautiful woman.
As time passed, the bear-now-woman grew sad--she was happy to be a human, but sad because she had no children. So again, Hwan-ung took heed of her pleas, and took her to be his wife. She gave birth to a handsome son named Tangun, who grew up to become a wise and powerful leader of the people on the peninsula. In 2333 BC, he moved to Pyongyang and founded Chosun, where he lived until he was 1,908 years old. Then he returned to Taebaeksan and became a sin-san or mountain god.
But the story doesn't end there. Miraculously, the North Korean government uncovered the burial site of King Tangun and erected a monument in 1994:
“Thanks to the correct policy of the Workers’ Party of Korea for preserving our nation’s cultural relics, Tangun, who was thought for thousands of years to have been a mythical being, was proved to have been a real person, and his tomb perfectly reconstructed in less than a year.”--from North Korea 1-on-1
I can't decide which part of the story is more fanciful.