The Netherlands was an important regional producer of natural gas and petroleum and played a major role as a transshipment center for mineral materials entering and leaving Europe— Rotterdam was the world's largest container port. The only other mineral of commercial importance was salt, and the only other mining operations left in the country were involved in the extraction of limestone, peat, and sand and gravel. The production of salt from the mines at Hengelo and Delfzijl was one of the oldest industries in the country; 5.0 million tons was produced in 2001 (various types), down from 5.5 in 1998. Because of oversupply and falling prices, Akzo Nobel Salt BV, the leading producer, was cutting capacity for its packaged salt in favor of its bulk grades. Magnesium chloride and oxide were produced in a plant at Veendam from extracted salt brines. Also produced in 2001 were hydraulic cement, nitrogen, industrial sand, sodium compounds, and sulfur. No metals were mined, but 3 million tons of iron ore was sintered from imported ore in 2001, down from 4.3 million tons in 1997. Coal was mined in Limburg until 1974. Among the country's leading industries in 2002 were metal products, chemicals, petroleum, and construction, and chemicals and fuels were top export commodities.