China's coastal ports enable the transportation of coal, containers, imported iron ore, and grain; roll-on-roll-off operations between mainland and islands; and deep-water access to the sea. In port construction, China has especially strengthened the container transport system, concentrating on the construction of a group of deep-water container wharves at Dalian, Tianjin, Qingdao, Shanghai, Ningbo, Xiamen and Shenzhen, and thus laying the foundations for China's container hubs. The coal transportation system has been further strengthened with the construction of a number of coal transport wharves. In addition, wharves handling crude oil and iron ore imports have been reconstructed or expanded. At the end of 2004, China's coastal ports had over 2,500 berths of medium size or above, of which 650 were 10,000-ton-class berths; their handling capacity was 61.5 million standard containers for the year, ranking first in the world. Freight volumes handled by some large ports exceed 100 million tons a year; and the Shanghai, Shenzhen, Qingdao, Tianjin, Guangzhou, Xiamen, Ningbo and Dalian have been listed among the world's top 50 container ports.