Standing atop the levee that protects Metairie, Louisiana, a satellite of New Orleans, from Lake Pontchartrain to the north, everything seems normal at first. But scanning your eyes across the horizon -- as I did last November, when I visited my hometown for Thanksgiving -- you suddenly glimpse the city's startling vulnerability. It's simply a question of elevation: On one side of the levee, the lake's water level comes up much higher than the foundations and baseboards of the nearby homes on the other side. Only the most expensive houses, those sporting third-story crow's nests, have rooftops that clear the levee's height.

In the event of a slow-moving Category 4 or Category 5 hurricane (with winds up to or exceeding 155 miles per hour), it's possible that only those crow's nests would remain above the water level. Such a storm, plowing over the lake, could generate a 20-foot surge that would easily overwhelm the levees of New Orleans, which only protect against a hybrid Category 2 or Category 3 storm (with winds up to about 110 miles per hour and a storm surge up to 12 feet). Soon the geographical "bowl" of the Crescent City would fill up with the waters of the lake, leaving those unable to evacuate with little option but to cluster on rooftops -- terrain they would have to share with hungry rats, fire ants, nutria, snakes, and perhaps alligators. The water itself would become a festering stew of sewage, gasoline, refinery chemicals, and debris.

I thought of the city's vulnerability recently, when the latest news came out from the National Oceanic and Atmospheric Administration: We can expect another very active Atlantic hurricane season this year, beginning on June 1 and stretching to the end of November. Last year, four hurricanes devastated swaths of Florida. One of the biggest ones, Ivan (a Category 4 storm) seemed to have New Orleans in its sights for a while. Ivan triggered a mass evacuation -- members of my family scrambled to Shreveport, Baton Rouge, and Houston -- but ultimately missed the city. Now, however, New Orleanians are in for another nail-biting fall and once again must contemplate the possibility of the dreaded "Atlantis scenario" becoming reality.

A direct hit from a powerful hurricane on New Orleans could furnish perhaps the largest natural catastrophe ever experienced on U.S. soil. Some estimates suggest that well over 25,000 non-evacuees could die. Many more would be stranded, and successful evacuees would have nowhere to return to. Damages could run as high as $100 billion. In the wake of such a tragedy, some may even question the wisdom of trying to rebuild the city at all. And to hear hurricane experts like Louisiana State University's Ivor van Heerden tell it, it's only a matter of time before the "big one" hits.

Currently, pretty much every long-term trend cuts against the safety of New Orleans. Levees are subsiding; coastal wetlands (which can slow storm surges) are continually disappearing; and sea levels are rising. And then there's global warming -- a warmer world with warmer ocean temperatures should theoretically experience worse hurricanes. Most importantly, the Atlantic Ocean appears to have entered an active hurricane cycle, with the potential to fling storms at the Gulf Coast for years to come. This puts New Orleans on the vanguard among U.S. coastal cities (including New York) that will have to think hard about their growing vulnerabilities in the coming years. The process of deciding how to save an entire coastal metropolis has begun, but the discussion has largely been confined to experts, and not nearly broad or ambitious enough yet.

It's time to make it that way -- before the next battery of hurricanes arrives, rather than afterward.

New Orleans already boasts some of the most powerful hurricane defenses in the world, yet the city will have to greatly amplify their strength. That engineering feat will take years, prompting talk of more radical short-term protections. Joseph Suhayda, a retired engineer and hurricane expert from Louisiana State University, has seriously proposed creating "community havens" by erecting massive concrete walls down the middle of New Orleans. In the event of a storm surge, the walls would protect hospitals and historic areas, even as those on the wrong side of them would remain unprotected. Where to build the wall would obviously pose a massive moral dilemma.

Meanwhile, the Army Corps of Engineers and others have considered the notion of armoring the I-10 twin span, near the mouth of Lake Pontchartrain, with a miles-long bulwark rising out of the water. If tall and strong enough, the sea wall, dubbed "Operation Block," would knock down any storm surge rising out of the Gulf of Mexico before it hit the lake -- in short, stopping a hurricane with concrete.

And that's just part of the multibillion-dollar program officials with the Corps have envisioned, which would include strengthening huge swaths of the Louisiana gulf coast. New Orleans would be the "only city in the country or even the world" with Category 5 hurricane protections, Corps' senior project manager Al Naomi told me last November. But these ideas are in little more than a brainstorming stage at this point; whether the bureaucratic Corps can lurch into action quickly enough to protect a city faced with ever increasing vulnerabilities remains a serious question. "Twenty years will be too late for New Orleans," says LSU's Van Heerden, who favors a specially funded congressional project more akin to the Tennessee Valley Authority.

Shockingly, even in the wake of the Asian tsunami catastrophe, there has been little widespread discussion of scenarios in which the United States could find part of its home territory devastated by the sea. Chatter in New Orleans itself has largely focused on improving evacuation plans and reducing gridlock as a storm approaches. These are necessary conversations to have, certainly, but bigger-picture perspectives have rarely surfaced in broader public discussions. That has to change -- and fast. Whatever other natural catastrophes we may be willing to tolerate, the possibility of losing an entire city, and especially the legendary (if also infamous) New Orleans, ought to be out of the question.

Chris Mooney is a Prospect senior correspondent whose TAP Online column appears each week. His book on the politicization of science will be published in September by Basic Books. His daily blog and other writings can be found at www.chriscmooney.com.