The Bakken Formation is a subsurface rock unit beneath about 200,000 square miles of the Williston Basin in Montana, North Dakota, Saskatchewan, and Manitoba. First described in 1953 and named for Henry O. Bakken, it consists of lower and upper shales surrounding a middle dolomite that serves as the main oil reservoir about two miles underground. Because the formation is unusually tight, its oil resisted economical production after its 1951 discovery until horizontal drilling and hydraulic fracturing became widely used after 2000. That technology helped make the Bakken a major source of U.S. oil, though the boom also brought transportation risks and other controversies.