Why was New Orleans so badly affected by Hurricane Katrina?
New Orleans is in a really vulnerable position for hurricanes. It lies above the Gulf of Mexico, where lots of the huge storms start.
The Mississippi river runs through the middle of town, and Lake Pontchartrain is to its north.
Because the city is on ground which is below sea level, these things combine to put it in a dangerous position for flooding.
What defences did the city have aginst flooding, and why didn't they work?
The river and the lake are higher than the city, but are kept in place by high banks of earth called levees.
These natural feaures have been built up, and the city also has a range of pumps to drain water away.
But the defences were only designed to hold off a direct hit from a category three hurricane.
Katrina was a category five, the strongest type. Although the centre of the hurricane didn't hit the city, floods caused by heavy rain broke down parts of the levees.
How bad is the damage?
Up to 80% of the city has been flooded, but experts say it would have been much worse if Katrina had directly hit New Orleans.
It's feared thousands of people have been drowned, and there are now worries that the water could spread disease among the survivors.
Why isn't the water going down?
Although New Orleans' defences didn't keep the water out, the high earth walls stopped the water from draining back into the river.
The pumps which normally help keep the city dry have also been damaged by the storm.
It's thought it could be months or years before New Orleans recovers.