The technique, also called fracking, has been widely used in the last decade to unlock vast new fields of natural gas, but drillers only recently figured out how to release large quantities of oil, which flows less easily through rock than gas. As evidence mounts that fracking poses risks to water supplies, the federal government and regulators in various states are considering tighter regulations on it.

The oil industry says any environmental concerns are far outweighed by the economic benefits of pumping previously inaccessible oil from fields that could collectively hold two or three times as much oil as Prudhoe Bay, the Alaskan field that was the last great onshore discovery. The companies estimate that the boom will create more than two million new jobs, directly or indirectly, and bring tens of billions of dollars to the states where the fields are located, which include traditional oil sites like Texas and Oklahoma, industrial stalwarts like Ohio and Michigan and even farm states like Kansas.

“It’s the one thing we have seen in our adult lives that could take us away from imported oil,” said Aubrey McClendon, chief executive of Chesapeake Energy, one of the most aggressive drillers. “What if we have found three of the world’s biggest oil fields in the last three years right here in the U.S.? How transformative could that be for the U.S. economy?”