The East China Sea is an arm of the Pacific Ocean which is bordered by the Chinese mainland, Yellow Sea, South China Sea, and the Philippine Sea. It is 290,000 square miles and rather shallow for a sea, with most of the depth less that 650 feet and an average depth of 1,145 feet. In the Okinawa Trough near the Ryukyu Islands the maximum depth reaches 8,912 feet. On the western edge, the continental shelf that extends between the South China and Yellow Seas continues which accounts for the shallowness.

Like the Yellow sea, the shallow parts of the bottom are covered with yellowish sediment from the Yangtze and other Chinese rivers. Many volcanos and volcanic islands can be found near the Ryukyu Island arc and a number of earthquakes occur in the area. The monsoon and typhoons are common in the area due to the Asian mainland being much warmer than the sea. Since the sea is constricted by the Yellow Sea and some of the inlets, the range of the tides around the coasts is very high.

Many countries fish in the East China Sea, including China, Japan, and North and South Korea, although it tends to be small local boats. A small portion on China's oil comes from the oil and natural gas deposits under the continental shelf. The sea is also home to main shipping routes between China, Korea, Japan, and other ports in the northern pacific. Since the 1950s, major research has been conducted on the East China sea predominately by the Chinese and sometimes the Japanese. This research has led to the discovery of some of the oil and gas reserves.

Sources:
http://www.eb.com/