North Dakota hugs the Canadian border in the upper Midwest of the United States. Fur trading was once the state’s primary economic activity until the construction of railroads led droves of Americans to come work the state’s rich soil. Agriculture has since become the driving force of North Dakota’s economy, spawning the creation of complementary food processing and farm equipment manufacturing industries. About 90% of North Dakota’s territory is covered by farmland. The top agricultural outputs of the state are wheat, cattle, soybeans, and corn. Mining of coal and petroleum also contribute a source of revenue to the state’s economy. The Bakken oil shale fields have one of the largest oil reserves in the United States, although some are skeptical about whether it is all recoverable by current technology.