Jeju

Jeju is the only special autonomous province of South Korea, situated on and coterminous with the country's largest island. Jeju-do lies in the Korea Strait, southwest of Jeollanam-do Province, of which it was a part before it became a separate province in 1946. Its capital is the city of Jeju.
The island contains the Natural World Heritage Site entitled Jeju Volcanic Island and Lava Tubes.

History
According to legend, three demi-gods emerged from Samsung-hyeol which is said to have been on the northern slopes of Mt. Halla and became the progenitors of the Jeju people who founded the Kingdom of Tamna. It has also been claimed that three brothers including Ko-hu who were the 15th descendants of Koulla, one of the Progenitors of the Jeju people, were received by the court of Silla at which time the name Tamna was officially recognized, while the official government posts of Seong-ju, Wang-ja and Do-nae were conferred by the count upon the three. While this was the golden period of Silla, the exact data is as yet unknown.

Although there is no concrete evidence of when the "Three Names" (Samseong-Ko, Yang and Pu) appeared nor for the exact date of when Ko-hu and his brothers were received by Silla. It may be supposed that the founding Period by the "Three Names" occurred during the Three Kingdoms (Goguryeo, Baekje and Silla) period on the peninsula.

Taejo, the foundoms (Goguryeo, Baekje and Silla) period on the same relationship to Goryeo as Tamna had been in its relation to Silla but with Tamna's refusal to accept this position the Goryeo court ordered a dispatch of army troops whereupon the chief of Tamna, Ko ja-gyeon, submitted to Goryeo and ordered his son. Prince Mallo, to Goryeo's court in 938. In 1105 (King Sukjong's 10th year), the Goryeo court abolished the name Takna which had to this time been used and from that year on, the island was known as "Tamna-gun" (district) and Goryeo officials were sent to handle the affairs of the island.

More information : Wikipedia