43. Harold Hamm & family
The 13th child of Oklahoma sharecroppers, he used to pick cotton and began working at a gas station at age 16 to support his family. He eventually started his own trucking company, hauling water to and from oilfields, then in 1971 took out a loan to drill his first well. In the 1990s, he had the vision to use horizontal drilling and hydraulic fracturing in North Dakota’s Bakken region, helping to transform the U.S. oil industry. Today his Continental Resources produces more than 200,000 barrels per day from North Dakota and Oklahoma. Hamm owns 75% of the company.