Busan is South Korea's second largest metropolis after Seoul, with a population of around 3.6 million. It is the largest port city in South Korea and the fifth largest port in the world. The city is located on the southeasternmost tip of the Korean peninsula and faces the Korea Strait. The most densely built up areas of the city are situated in a number of narrow valleys between the Nakdong River and Suyeong River, with mountains separating some of the districts. Nampodong to the south is Busan's shopping and entertainment downtown, while central Seomyeon at the intersection of subway lines 1 and 2 is the main office building area. Between them are Busan's train station and its international ferry terminals. The beaches of Gwangalli, Haeundae and Songjeong lie to the east, the ruins of mountain fortress Geumjeong guard the north. To the west is Gimhae town where the Busan Airport is located.