Bakken Shale

Hours from Standing Rock, N.D., is another reservation, in the the Bakken oilfield's sweet spot. Drilling has brought in millions of dollars, but the tribes have environmental worries, too.

Oil worker Dustin Bergsing, 21, was found dead on top of a North Dakota oil tank in 2012. A journalist and a doctor looking into the death found a pattern of similar fatal accidents.

Trains carrying oil from North Dakota pass through American towns daily — and sometimes they derail and explode. The oil industry is now under pressure to make the oil less volatile before shipping.