In Florence and the surrounding area, the first recognized oil field west of the Mississippi River was developed after A.M. Cassidy discovered oil in Fremont County in 1862. More than 1,300 wells have been drilled in this 14-square-mile area, averaging 2,300 barrels per acre, including many within the city limits. One mile north, well No. 42, drilled in 1889, is the oldest continuous commercial producing oil well in the world and has produced more than 1,000,000 barrels.