Underlying northwestern North Dakota is a massive rock formation, referred to as the Bakken shale, which holds an estimated 18 billion barrels of crude oil. When this resource was first discovered in 1951, recovering it was financially unfeasible because the oil was embedded in the stone. Then, around 2008, everything changed, and North Dakota boomed. New drilling technology called hydraulic fracturing, or "fracking," became widespread, and oil production took off. As of 2013, there are more than 200 active oil rigs in North Dakota, producing about 20 million barrels of oil every month -- nearly 60 percent of it shipped by rail, rather than pipeline. The rigs and support systems have resculpted the landscape, millions of dollars are being spent on infrastructure upgrades across the area, and thousands of oil field workers have arrived, living in new or temporary housing. Gathered below is a collection of images of this recent boom, spread across North Dakota's wide open plains.