Shortly after Hurricane Katrina slammed into the Louisiana coast, flooding the city of New Orleans, journalists began reporting on a “toxic soup” of chemicals and dangerous microbes bathing the city. Based on no reported data, these stories nevertheless seemed reasonable; the city’s sewer system had flooded, and thousands of cars, houses, and chemical storage tanks lay beneath water, which in part of the city reached more than 3 meters in depth. In addition, 24 Superfund sites are in the affected area, and the U.S. EPA and the U.S. Coast Guard have tallied more than 400 oil and hazardous chemical spills....