Hurricane Katrina, the costliest natural disaster in United States history, was also a warning shot. Right after the tragedy, many people expressed a defiant resolve to rebuild the city. But among engineers and experts, that resolve is giving way to a growing awareness that another such disaster is inevitable, and nothing short of a massive and endless national commitment can prevent it.

Located in one of the lowest spots in the United States, the Big Easy is already as much as 17 feet (five meters) below sea level in places, and it continues to sink, by up to an inch (2.5 centimeters) a year. Upstream dams and levees built to tame Mississippi River floods and ease shipping have starved the delta downstream of sediments and nutrients, causing wetlands that once buffered the city against storm-driven seas to sink beneath the waves. Louisiana has lost 1,900 square miles (4,900 square kilometers) of coastal lands since the 1930s; Katrina and Hurricane Rita together took out 217 square miles (562 square kilometers), putting the city that much closer to the open Gulf. Most ominous of all, global warming is raising the Gulf faster than at any time since the last ice age thawed. Sea level could rise several feet over the next century. Even before then, hurricanes may draw ever more energy from warming seas and grow stronger and more frequent.

And the city's defenses are down. Despite having spent a billion dollars already, the U.S. Army Corps of Engineers now estimates it will take until after 2010 to strengthen the levee system enough to withstand a 1-in-100-year storm, roughly the size of Category 3 Katrina. It would take decades more to protect the Big Easy from the truly Big One, a Category 4 or 5—if engineers can agree on how to do that and if Congress agrees to foot the almost unimaginable bill. For now, even a modest, Category 2 storm could reflood the city.

The long odds led Robert Giegengack, a geologist at the University of Pennsylvania, to tell policymakers a few months after the storm that the wealthiest, most technologically advanced nation on the globe was helpless to prevent another Katrina: "We simply lack the capacity to protect New Orleans." He recommended selling the French Quarter to Disney, moving the port 150 miles (240 kilometers) upstream, and abandoning one of the most historic and culturally significant cities in the nation. Others have suggested rebuilding it as a smaller, safer enclave on higher ground.

But history, politics, and love of home are powerful forces in the old river town. Instead of rebuilding smarter or surrendering, New Orleans is doing what it has always done after such disasters: bumping up the levees just a little higher, rebuilding the same flood-prone houses back in the same low spots, and praying that hurricanes hit elsewhere. Some former New Orleanians may have had enough. More than a third of the city's pre-Katrina population has yet to return. Those who have face deserted neighborhoods, surging crime, skyrocketing insurance, and a tangle of red tape—simply to rebuild in harm's way.

IF PARIS, AS HEMINGWAY SAID, is a movable feast, then New Orleans has always been a floating one. Born amid willow and cypress swamps atop squishy delta soils, the city originally perched on the high ground formed by over-wash deposits from annual river floods. Jean-Baptiste Le Moyne, Sieur de Bienville, actually had to wait for the water to recede before he could plant the French flag in 1718. A flood destroyed the village the year after he founded it, and hurricanes wiped it off the map in 1722 and again a year later. In its 289-year history, major hurricanes or river floods have put the city under 27 times, about once every 11 years. Each time, the fractious French, Spanish, blacks, Creoles, and Cajuns raised the levees and rebuilt.