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Twenty years after the Exxon Valdez supertanker spilled oil into Alaska's Prince William Sound there is still oil around, Nature Geoscience reports. The massive cleanup effort that took place did not get rid of everything. Some oil washed onto beaches and seeped through a porous top layer of sediment and then stuck in a lower layer where it is having difficulty biodegrading. Otters and other animals that dig along the shoreline can bring the oil back up. Scientists say that injecting chemicals to promote biodegradation in the bottom layer could be a cleanup solution.