Jeju Island is a volcanic island, dominated by Halla-san (Halla Mountain): a volcano 1,950 metres high and the highest mountain in South Korea.  The island was created entirely from volcanic eruptions approximately 2 million years ago.

Some tourist attractions are water sports, jet ski, snorkling, etc.  You can take submarine ship and enjoy the underwater view.

The [...]

Mt. Sorak or Seoraksan is the highest mountain in the Taebaek mountain range (태백산맥) in the Gangwon province in eastern South Korea. It is located in a national park near the city of Sokcho. After the Hallasan volcano on Jeju Island and Jirisan in the south, Seoraksan is the third highest mountain in South Korea. [...]