arm of the western Pacific Ocean that borders the Southeast Asian mainland. It is bounded on the northeast by the Taiwan Strait (by which it is connected to the East China Sea); on the east by Taiwan and the Philippines; on the southeast and south by Borneo, the southern limit of the Gulf of Thailand (Thailand, Gulf of), and the east coast of the Malay Peninsula; and on the west and north by the Asian mainland. The South China Sea and the East China Sea together form the China Sea. The southern boundary of the South China Sea is a rise in the seabed between Sumatra and Borneo, and the northern boundary stretches from the northernmost point of Taiwan to the coast of Fujian province, China, in the Taiwan Strait. It embraces an area of about 1,423,000 square miles (3,685,000 square km), with a mean depth of 3,976 feet (1,212 metres).

The sea's major feature is a deep rhombus-shaped basin in the eastern part, with reef-studded shoals rising up steeply within the basin to the south (Reed and Tizard banks and the Nanshan Island area) and northwest ( Paracel Islands and Macclesfield banks). The deep portion, called the China Sea Basin, has a maximum depth of 16,457 feet (5,016 metres) and an abyssal plain with a mean depth of some 14,100 feet (4,300 metres).

The Sunda Shelf is covered with littoral sediments contributed by submerged valleys. The inner zone of mud is characteristic of the continental shelf near the Mekong (Mekong River) and Red (Red River) river deltas, while the sediment of the deeper parts of the South China Sea is mainly composed of clay. A characteristic part of the sediments in both deep and shallow water is volcanic ash. This is found in layers, derived from large volcanic eruptions in the East Indies, notably the enormous eruption of Krakatoa in 1883, during which ash was transported through the entire area by both wind and currents.

Monsoons control the sea-surface currents as well as the exchange of water between the South China Sea and adjacent bodies of water. In August the surface flow into the South China Sea is from the south from the Java Sea through the Karimata and Gelasa (Gasper) straits. Near the mainland the general flow is northeasterly, passing out through the Taiwan and Luzon straits. There is a weak countercurrent on the eastern side of the sea. In February the flow is generally to the southwest; the strongest flow occurs in summer along the bulging part of Vietnam, with speeds of up to 3.5 miles (5.6 km) per hour generated by the strong southwestern monsoon.

The South China Sea is rich in marine life. Contributing to this abundance are the extensive runoff of nutrient-laden waters from land and the upwellings of water in certain areas of the sea. The sea is heavily fished, however, and is the main source of animal protein for the densely populated Southeast Asian area. Most abundant are the various species of tuna, mackerel, croaker, anchovy, shrimp, and shellfish. Nearly the entire catch is consumed locally, either fresh or preserved.

The South China Sea contains some of the world's most important shipping lanes. The main route to and from Pacific and Indian ocean ports is through the Strait of Malacca and the South China Sea. Generally, oil and minerals move north, and food and manufactured goods move south. Some areas in the central South China Sea are not well sounded, and nautical charts bear the notation “dangerous ground.” More recently, a “dangerous” designation has also arisen from international territorial disputes, especially over the Spratly Islands, which lie in the oil-rich zone between Malaysia, Brunei, the Philippines, and Vietnam; those four countries, plus China and Taiwan, have laid some claim to the group.

For the South China sea, see Dennis E. Hayes (ed.), The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, 2 vol. (1980–83); Choon-Ho Park, East Asia and the Law of the Sea (1983); and Douglas M. Johnston, Environmental Management in the South China Sea: Legal and Institutional Developments (1982). Eugene C. LaFond