For years, forecasters have warned of the nightmare flooding a big storm could bring to New Orleans, a bowl-shaped city bounded by the half-mile-wide Mississippi River and massive Lake Pontchartrain. As much as 10 feet below sea level in spots, the city is as the mercy of a network of levees, canals and pumps to keep dry.

Scientists predicted Katrina could easily overtake that levee system, swamping the city under a 30-feet cesspool of toxic chemicals, human waste and even coffins that could leave more than 1 million people homeless.

"All indications are that this is absolutely worst-case scenario," Ivor van Heerden, deputy director of the Louisiana State University Hurricane Center, said Sunday afternoon.