By the time a contractor spotted a burst in the wall of an Alberta tar sands pipeline on July 15, a spill was already well underway. In a public apology on Friday, the pipeline’s owner, Nexen Energy, announced the spill was contained, but the cause, and the length of time it had been spilling, was still unknown. Thirty-one thousand barrels (roughly 1.3 million gallons) of bitumen, water and sand had spilled—a greater volume than even the 2010 Kalamazoo River spill in Michigan, still the largest land spill in the U.S.