The controversial pipeline operated by TransCanada started leaking Thursday outside the town of Amherst, South Dakota.
It is the largest Keystone oil spill in state history, 12 times bigger than a leak last year that took two months to clean up.
And this leak came just days before Nebraska lawmakers announce their decision on a sister Keystone Pipeline project.
The two oil transportation systems have drawn opposition in the past as potential environmental hazards.
TransCanada officials say the pipeline is under control and there is no threat to public safety, although some of the oil made it above ground and it will take a while before they know if the underground leak contaminated groundwater.