The most important resource of the 20th century, oil and its by-products have had many uses throughout history. Oil-burning lamps are believed to have been used by the Sumerians as early as 3000 BC. Petroleum, meanwhile, was used almost exclusively as a medicine until the 19th century when its use as a clean and cheap lighting source was revealed. A decade after the 1849 gold rush, an oil boom in Pennsylvania sent prospectors east. Prospecting for oil in the  middle-eastern countries began shortly thereafter.