A series of oil rigs in the Gulf of Mexico, damaged during a September 2004 hurricane, are still leaking in a "chronic" oil spill. SkyTruth brought the pollution to light using satellite imagery, aerial photography, and publicly available government data. The slicks are bigger than the company responsible is reporting.
"Chronic Oil Spill in the Gulf of Mexico Since 2004 -- Still Spilling"
Source: SkyTruth, 06/05/2013