By Helen Lambourne
Researcher, BBC Horizon
In the chaos that followed the worst natural disaster in American history, a forensic investigation has been taking place to find out what went wrong and why.
Venice from hell: New Orleans' flood system was shattered
The BBC's Horizon programme has spoken to the scientists who are now confronting the real possibility that New Orleans may be the first of many cities worldwide to face extinction.
Modern day New Orleans was a city that defied the odds. Built on a mosquito-infested swamp squashed between two vast bodies of water in what is essentially a bowl, its very existence seemed proof of the triumph of engineering over nature.
But on the 29 August 2005, New Orleans took a hit from Hurricane Katrina and overnight was turned into a Venice from hell.
The delicate flood system in New Orleans, which so many relied on to protect them, was actually, year on year, adding to the risk of a catastrophe in the city.
Coastal Geologist Shea Penland, from the University of New Orleans, knows every inlet, every cove and every stretch of marsh that surrounds New Orleans.
He also knew that what had been thought of as wasteland for years was critical to the survival of the city.
"The first line of defence isn't the levee in your backyard, the first line of defence is that marsh in your backyard and we're learning what that means," he said.
The Mississippi River had been controlled over the years to stop the annual floods with hundreds of kilometres of levees and dams. As a result, sediments that were naturally brought down to replenish the land were cut off.
Gradually Louisiana started to lose its coast and today it has the highest rate of coastal land loss in North America. An area the size of Wembley stadium is lost to the sea every 20 minutes.
Professor Penland has no doubt about why the hurricane was so devastating.
New Orleans highlighted the fragile balance of living behind levees
"What we see, just played out there in the summer 2005 hurricane season, was the consequences of river control, living behind levees, living in a walled city where we have to pump water up hill to get it out. How long can you live in a bowl?" he said.
The loss of sediment to build up the land has led to another problem.
Much of the city is below sea level and continual pumping has caused the ground to subside. Since 1878 the city has sunk by 4.6m (15ft), one of the highest rates of subsidence in the entire United States.
Geologist Professor Harry Roberts has spent the past 20 years watching his city sink.
"When you pump the water out of those kinds of soils they start to collapse and more importantly the organic material oxidises and goes away; so you've taken out one component of the soil, and all that adds up to subsidence," he said.
At the earliest opportunity after Katrina had passed, Shea Penland chartered a seaplane to investigate the overnight loss to Louisiana's precious wetlands.
What he discovered sounded like the death knell for the city. In just one night, Louisiana had lost about 3,900 hectares (15 sq miles) of wetlands, three-quarters of its annual loss in 24 hours.
"If you want New Orleans back you have to do some very fundamental things," said Professor Penland.
New Orleans' wetlands are crucial to its future survival, say scientists
"You're going to have to bring the land back that protects the city from the ravages of hurricanes. If we don't incorporate that then the city will be faced with extinction."
Local urban planners believe that the survival of the city is dependent on preserving its lowest lying areas, its devastated residential areas, as parkland.
Areas like the Lower Ninth Ward built 2.4m (8ft) below sea-level - and where hundreds of people died - may not be part of the city's future.
Instead, they could be turned into green spaces, serving both as buffers against future flood waters and as a reminder that nature sometimes should be left alone.
Even if some residential areas are not rebuilt, the city will still need hugely increased defences. In order to assess that task, engineers at Louisiana State University have investigated why the floods were so devastating.
By collecting residents' eye-witness testimonies and stopped clocks from their flooded homes, Professor Ivor Van Heerden and his team has been able to piece together a timeline of the levee breaches.
Their initial results suggest the levees were breached while the waters were still rising. They found faulty design in the levees and weakness in the soils underneath.
According to Professor Van Heerden: "The system wasn't even capable of withstanding a Category One hurricane."
To make New Orleans safe to withstand a Category Five hurricane, his proposal is for a vast barrier system stretching from Mississippi all the way to Texas.
It could take 20 years to build but Van Heerden believes this is the only way to guarantee the safety of the city's people.
Without adequate protection, the future looks dismal for New Orleans.
The future of this sinking city is further compounded by the effects of global warming with its attendant raised sea-levels and the potential for hurricanes of increased intensity.
"New Orleans' future is very hard to predict," said Professor Van Heerden. "The big unknown is global warming. If sea level rises come up by another metre in the next 50 to 60 years, if we see far more of these major hurricanes, we could well reach a point where we see we need to abandon these cities like New Orleans."
Horizon: The Lost City of New Orleans was shown on BBC One on Thursday 2 February 2006.