Greater Sudbury is the largest city in Northern Ontario. Sudbury was founded in 1883 following the discovery of nickel ore during the construction of the transcontinental railway. The people live in an urban core and many smaller communities scattered around three hundred lakes and among hills of rock blackened by smelting activity. Mining and related industries dominated the economy for much of the twentieth century. The two major mining companies which shaped the history of Sudbury were Inco, now Vale Limited, which employed more than 25% of the population by the 1970s, and Falconbridge, now Glencore. Sudbury has since expanded from its resource-based economy to emerge as the major retail, economic, health and educational centre for North-eastern Ontario.The city recovered from the Great Depression much more quickly than almost any other city in North America due to increased demand for nickel in the 1930s. Sudbury was the fastest-growing city and one of the wealthiest cities in Canada for most of the decade. Many of the city's social problems in the Great Depression era were caused by the difficulty in keeping up with all of the new infrastructure demands created by rapid growth. Employed mineworkers sometimes ended up living in boarding houses or makeshift shanty towns because demand for new housing was rising faster than supply.The open coke beds used in the early to mid-twentieth century and logging for fuel resulted in almost a total loss of native vegetation in the area. Consequently, the terrain was made up of exposed rocky outcrops permanently stained charcoal black, first by the air pollution from the roasting yards. Acid rain added more staining, in a layer that penetrates up to three inches into the once pink-grey granite.The construction of the Inco Super stack in 1972 dispersed sulphuric acid through the air over a much wider area, reducing the acidity of local precipitation. This enabled the city to begin an environmental recovery program. In the late 1970s, private and public interests combined to establish a "regreening" effort. Lime was spread over the charred soil by hand and by aircraft. Seeds of wild grasses and other vegetation were also spread. More than nine million new trees have been planted in the city.Sudbury's pentlandite, pyrite and pyrrhotite ores contain profitable amounts of many elements-primarily nickel and copper, but also platinum, palladium and other valuable metals.