South Korea, officially the Republic of Korea, is a country in East Asia, constituting the southern part of the Korean Peninsula, and sharing a land border with North Korea. 25 million people, around half of the country's population of more than 51 million people, live in the Seoul Capital Area, the fifth-largest metropolitan area in the world. The Korean Peninsula was inhabited as early as the Lower Paleolithic period. Its first kingdom was noted in Chinese records in the early 7th century BCE. Following the unification of the Three Kingdoms of Korea into Silla and Balhae in the late 7th century, Korea was ruled by the Goryeo dynasty and the Joseon dynasty. The succeeding Korean Empire was annexed into the Empire of Japan in 1910. After World War II, Korea was divided into two zones; a northern zone occupied by the Soviet Union and a southern zone occupied by the United States.