Exxon Mobil said it expected to pay more than $100 million in cleanup operations for the July 2011 rupture of the Silvertip oil pipeline near Billings, Mont. An estimated 1,200 barrels spilled from the pipeline into the Yellowstone River.
Tom Livers, deputy director of the Montana Department of Environmental Quality, said responders had never dealt with a spill of that size.
"We were kind of learning as we went on some of this," he was quoted by The Billings (Mont.) Gazette as saying.
Montana Gov. Brian Schweitzer called for a review of the 9,000 pipelines that cross waterways in the state. Scouring of the pipeline, which was around 4 feet below the river bottom, is suspected of contributing to the rupture of Silvertip.
Exxon has since bored holes under the river to bury the pipeline deeper beneath the surface. U.S. pipeline safety regulators are still investigating the formal cause of the spill.
Notable deaths of 2014 [PHOTOS]