S1, E2: Giant swamp rats are taking over the U.S. and destroying fragile ecosystems from Maryland to Oregon. The rodent, known as nutria, is native to Argentina, but was introduced to the U.S. during a "get-rich-quick" scheme to farm the animals for their fur. Now there are 20 million in Louisiana alone, and Nat Geo WILD is on the front lines with experts trying to curtail the invasive species.