The search for petroleum in Illinois occurred years before the boom days of the early 1900s. Wells were first drilled in 1865 near the town of Casey and then in other areas within the state for the next 40 years with very limited success. Renewed interest in the Casey area resulted in an oil discovery in 1905, followed by significant discoveries in Crawford and Lawrence Counties in 1906, pushing the state's oil production that year to 4.4 million barrels. Towns near the oil fields, such as Oblong, Robinson, Casey, Lawrenceville, and Bridgeport, experienced rapid population growth. Oil refineries near Chicago and St. Louis and in southeastern Illinois provided additional employment. The discovery of the Salem oil field in Marion County in 1938 initiated a major oil boom. By 1940, Illinois ranked third in the nation in oil production, providing a boast to the Allied forces' effort in Europe. Illinois continues to contribute to the petroleum industry of the United States, ranking fourth in crude oil refining and 16th in oil production.