Black Diamond Mines Regional Preserve was the site of California's largest coal field. From the mid-1850s to the early 1900s, a dozen mines supplied nearly four million tons of coal to the expanding urban and industrial centers of the San Francisco Bay Area. Mount Diablo coal was an economical substitute for coal shipped from Australia, Chile, and England and was extensively used to power mills, factories, and river boats. People from all over the world were drawn to the area's five mining towns. As many as 900 miners, some as young as eight years old, labored in hundreds of miles of underground workings. At the peak of operations, the coal field was reported to have been the population center of Contra Costa County. Significant production ended in 1902 as a result of higher mining costs, competition from new mines in Washington Territory, and the advent of oil as an industrial power source.