outh Korea, officially the Republic of Korea (ROK), is a country in East Asia, constituting the southern part of the Korean Peninsula and sharing a land border with North Korea. Its western border is formed by the Yellow Sea, while its eastern border is defined by the Sea of Japan. South Korea can be divided into four general regions: an eastern region of high mountain ranges and narrow coastal plains; a western region of broad coastal plains, river basins, and rolling hills; a southwestern region of mountains and valleys; and a southeastern region dominated by the broad basin of the Nakdong River. South Korea is home to three terrestrial ecoregions: Central Korean deciduous forests, Manchurian mixed forests, and Southern Korea evergreen forests. South Korea's terrain is mostly mountainous, most of which is not arable. Lowlands, located primarily in the west and southeast, make up only 30% of the total land area. About three thousand islands, mostly small and uninhabited, lie off the western and southern coasts of South Korea.