Five years after the worst accidental marine oil spill ever, scientists are untangling how the explosion of the Deepwater Horizon oil rig has left its mark on the Gulf of Mexico. In the Louisiana marshlands, they are finding signs of continued damage and remarkable resilience. Although oil is visible only in spots, toxic residues could linger for decades. Organisms from bacteria to dolphins show signs of the oil's effects. Yet fish appear to be flourishing, brown pelican numbers have remained steady, and insects seem to be on the rebound. Scientists wonder whether the worst has passed, or surprises lie ahead.