The U.S. Geological Survey keeps increasing its estimate for the amount of oil under North Dakota. In 2008, the organization estimated that oil deposits in part of the Williston Basin—an area that includes parts of North Dakota, South Dakota, and Montana—had 3.65 billion barrels of oil yet to be discovered. That was 25 times higher than its previous estimate, made in 1995, of about 150 million barrels. Now it’s increased its estimate by a similar amount, raising it 3.75 billion barrels to 7.4 billion barrels. The total is a little more than the amount the United States consumes in one year.

Why is the estimate of the amount of oil going up, even as oil producers do their best to suck that oil out of the ground? Why aren’t the numbers actually going down as that oil is depleted?

There are two main reasons. The first is that geologists’ understanding of oil-containing formations is incomplete. As producers drill more wells, the geologists learn more about the formation.

The second is that the estimate isn’t actually for the total amount of oil underground, but just the amount of oil that can be recovered with existing technology. As technology improves, more oil can be accessed, and the estimate goes up.

The increased estimate this time is because producers have figured out how to access oil in much deeper formations in North Dakota, opening up a huge new potential resource. The 2008 increase had to do with the techniques of hydraulic fracturing and horizontal drilling that made it possible to get oil out of formations that had previously been thought to trap oil too tightly for it to be extracted.