The Keystone Pipeline suffered the worst leak in its history last week. Nearly 600,000 gallons of oil spilled into surrounding waterways and land. EPA said the oil that was spilled was diluted bitumen, which is complicating the cleanup efforts. Bitumen is a dense, thick form of oil that shippers dilute with lighter oils so it can move easier through pipelines. Bitumen sinks in water which makes it difficult to collect. Additionally, diluted bitumen forms a residue that sticks to surfaces. So far the response team has been able to recover 5,567 of the 14,000 barrels of oil spilled. The bodies of four mammals and 71 fish that perished in the spill have been recovered.