April 20 marks the three-year anniversary of the Deepwater Horizon disaster, which took the lives of 11 men and resulted in the largest oil spill in American history. BP, along with Transocean and Halliburton, are still in the midst of a civil trial held in New Orleans federal court over liability for the catastrophe.

The extent of the damage and the long-term effects from the spill remain impossible to determine. Some scientific evidence -- for example, that collected by NOAA's Damage Assessment, Remediation, and Restoration Program, which includes the results of necropsies of dead sea turtles and dolphins -- is not available, since it is being used as evidence in the trial. Yet even three years later, the residual effects of the oil spill are still apparent on the Gulf Coast. I covered the BP oil spill from the start, and have gone on documenting the effects of the hardest-hit areas in Louisiana and Mississippi, revisiting those areas over the last week. Below are some of the photos I have taken. Along the Mississippi coast one can still find tar balls. In Louisiana I observed, among other disturbing signs of the spill, oil sheen along a coastal marsh, and erosion on an island in Barataria Bay sped up by the death of mangrove trees and marsh grass.

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