By land

Due both to its location at the end of the Korean peninsula and the political situation with North Korea, entering South Korea overland is practically not possible. The border between North and South Korea is considered the most heavily fortified border in the world, and while some crossings have occurred at the truce village of Panmunjeom, one of the cases (a Soviet defector in 1984) was shot at by both sides and, although he survived, you might not be so lucky. In the 80's and the early 90's most of those who crossed the border either way would be arrested and prosecuted for reasons mostly referred to as 'threatening national security'.

See Also Get in By plane By train By boat