I just found this in the TalkLeft archives, from September, 2004, when it looked like Hurricane Ivan was headed to New Orleans. ABC News reported that New Orleans might sink if it got hit hard:

The worst-case scenario for New Orleans a direct strike by a full-strength Hurricane Ivan could submerge much of this historic city treetop-deep in a stew of sewage, industrial chemicals and fire ants, and the inundation could last for weeks, experts say. If the storm were strong enough, Ivan could drive water over the tops of the levees that protect the city from the Mississippi River and vast Lake Pontchartrain. And with the city sitting in a saucer-shaped depression that dips as much as 9 feet below sea level, there would be nowhere for all that water to drain.

Those folks who remain, should the city flood, would be exposed to all kinds of nightmares from buildings falling apart to floating in the water having nowhere to go," Ivor van Heerden, director of Louisiana State University's Hurricane Public Health Center, said Tuesday.