Taiwan, officially the Republic of China (ROC), is an island state in East Asia. The main island of Taiwan, known historically as Formosa, makes up 99% of the area controlled by the ROC, measuring 35,808 square kilometres (13,826 sq mi) and lying some 180 kilometres (112 mi) across the Taiwan Strait from the southeastern coast of mainland China. The East China Sea lies to its north, the Philippine Sea to its east, the Luzon Strait directly to its south and the South China Sea to its southwest. Smaller islands include a number in the Taiwan Strait including the Penghu archipelago, the Kinmen and Matsu Islands near the Chinese coast, and some of the South China Sea Islands. The main island is a tilted fault block, characterized by the contrast between the eastern two-thirds, consisting mostly of five rugged mountain ranges parallel to the east coast, and the flat to gently rolling plains of the western third, where the majority of Taiwan's population reside. There are several peaks over 3,500 m, the highest being Yu Shan at 3,952 m (12,966 ft), making Taiwan the world's fourth-highest island. The tectonic boundary that formed these ranges is still active, and the island experiences many earthquakes, a few of them highly destructive. There are also many active submarine volcanoes in the Taiwan Straits. The climate ranges from tropical in the south to subtropical in the north, and is governed by the East Asian Monsoon. The main island is struck by an average of four typhoons in each year. The eastern mountains are heavily forested and home to a diverse range of wildlife, while land use in the western and northern lowlands is intensive.