This summer, an Exxon Mobil pipeline carrying oil across Montana burst suddenly, soiling the swollen Yellowstone River with an estimated 42,000 gallons of crude just weeks after a company inspection and federal review had found nothing seriously wrong.
 

And in the Midwest, a 35-mile stretch of the Kalamazoo River near Marshall, Mich., once teeming with swimmers and boaters, remains closed nearly 14 months after an Enbridge Energy pipeline hemorrhaged 843,000 gallons of oil that will cost more than $500 million to clean up.

While investigators have yet to determine the cause of either accident, the spills have drawn attention to oversight of the 167,000-mile system of hazardous liquid pipelines crisscrossing the nation.