Subsistence agriculture supports most of the population; sweet potatoes constitute the main food crop. Agricultural exports (notably palm oil, coffee, cocoa, coconut products, rubber, and tea) are increasing, but mineral and oil deposits account for the majority of export earnings. Copper, gold, and silver are mined, oil production began in 1992, and there are undeveloped natural gas reserves. Timber is another import source of revenue, but logging, largely by foreign companies, is often done without regard for laws designed to promote sustainable yields from the country's rain forests. Pearl-shell and tortoise fisheries dot the coast, and crayfish and prawns are exported. Most industry involves the processing of agricultural and wood products; there is also petroleum refining, construction, and some tourism. Machinery and transportation equipment, manufactured goods, food, fuels, and chemicals are imported. Australia is by far the largest trading partner, followed by Singapore and Japan.