The worst-case scenario for New Orleans (search) — a direct strike by a full-strength hurricane — 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, it could drive water over the tops of the levees that protect the city from the Mississippi River and vast Lake Pontchartrain (search). 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.

Even in the best of times, New Orleans depends on a network of canals and huge pumps to keep water from accumulating inside the basin.

LSU's hurricane experts have spent years developing computer models and taking surveys to predict what might happen.

Computer models show a hurricane with a wind speed of around 120 mph or more — hitting just west of New Orleans so its counterclockwise rotation could hurl the strongest surf and wind directly into the city — would push a storm surge from the Gulf of Mexico and Lake Pontchartrain (search) over the city's levees.

New Orleans would be under about 20 feet of water, higher than the roofs of many of the city's homes.

Severe flooding in area bayous also forces out wildlife, including poisonous snakes and stinging fire ants, which sometimes gather in floating balls carried by the current.

Much of the city would be under water for weeks. And even after the river and Lake Pontchartrain receded, the levees could trap water above sea level, meaning the Army Corps of Engineers would have to cut the levees to let the water out.

Experts say that if the eye were to come ashore east of the city, New Orleans would be on the low side of the storm surge and would not likely have catastrophic flooding.

The worst storm in recent decades to hit New Orleans was Hurricane Betsy in 1965, which submerged parts of the city in water 7-feet deep and was blamed for 74 deaths in Louisiana, Mississippi and Florida. That storm was a Category 3.

Even if New Orleans escapes this time, it will remain vulnerable until the federal and state governments act to restore the coastal wetlands that should act as a buffer against storms coming in from the Gulf.

Louisiana has lost about a half million acres of coast to erosion since 1930 because the Mississippi River is so corralled by levees that it can dump sediment only at its mouth, and that allows waves from the Gulf to chop away at the rest of the coastline.