Taiwan perches on the margin of the continental shelf. Along the west coast the sea is rather shallow, averaging 90 m (300 ft) and not exceeding 210 m (690 ft) at the deepest point; however, it deepens abruptly along the east coast, dropping to a depth of 4,000 m (13,000 ft) only 50 km (31 mi) offshore. The terrain is precipitous on the east coast, with practically no natural harbor except Suao Bay in the north. The west coast is marked by wide tidal flats. Kaohsiung, the southern port, is situated in a long lagoon called Haochiung Bay. The north coast with its many inlets provides Taiwan with its best harbor, Chilung (Keelung).
The eastern two-thirds of the island is composed of rugged foothill ranges and massive mountain chains. A low, flat coastal plain, extending from north to south, occupies the western third. Yü Shan, with an elevation of 3,997 m (13,113 ft), is the highest peak on the island. Mild earthquake tremors are common.
All the rivers originate in the mountains in the central part of the island. They have short courses and rapid streams. The longest river, Choshui, draining westward, is only 190 km (118 mi) long. Only the Tanshui, which flows past T'aipei in the north, is navigable.