The Korean peninsula is largely mountainous; the principal series of ranges, extending along the east coast, rises (in the northeast) to 9,003 ft (2,744 m) at Mt. Paektu (Baekdu), the highest peak in Korea. Most rivers are relatively short and many are unnavigable, filled with rapids and waterfalls; important rivers, in addition to the Yalu and Tumen, are the Han, the Geum, the Taedong (Daedong), the Nakdong, and the Seomjin. Off the heavily indented coast (c.5,400 mi/8,690 km long) lie some 3,420 islands, most of them rocky and uninhabited (of the inhabited islands, about half have a population of less than 100); the main island group is in the Korean Archipelago in the Yellow Sea. The climate of Korea ranges from dry and extremely cold winters in the north to almost tropical conditions in parts of the south.
Many Koreans are Buddhists or Confucianists, although the people tend to be eclectic in their religious practices. Korean Confucianism, for example, has developed into more of an ethical system than a religion, and its influence is wide and pervasive. Of the various indigenous religions, Chondogyo (a native mixture of Buddhism, Confucianism, and Taoism) is the most influential. South Korea has a large number of practicing Christians, roughly a quarter of the population. (Roman Catholicism was introduced in the late 18th cent., and Protestantism in the late 19th cent.) The North Korean government has actively suppressed religion as contrary to Marxist belief. Korean is spoken in both countries, and English is widely taught in South Korean schools.