The geography of southeastern Louisiana is unlike any place else on Earth. Much of what looks like solid ground on a map is actually marshland, floating like a pancake on a plateful of syrup.
Scientists are now piecing together how Hurricane Katrina affected those marshes, which form a buffer against storms and flooding. What they find will help determine how the region is rebuilt. For the latest NPR/National Geographic Radio Expedition report, Christopher Joyce journeys to "liquid Louisiana" to survey the damage.
Scientists believe Hurricane Katrina created a giant storm surge that gathered in the Gulf of Mexico and barreled westward up the wide swampy delta on its way to New Orleans. It may have reached 20 feet high by the time it hit the city's eastern suburbs.
Levees built to protect the city may have actually focused that storm surge: Instead of spreading a sheet of water out across the delta, the levees created a channel for the surge. Also, the natural marsh buffer zones that soften the blow of a storm surge have been largely replaced or hemmed in by ship channels and development. All those channels and levees cut off river sediment that enable the marsh to take root and thrive.
What happened to the city is now well known. But damage to the marsh is harder to evaluate. From the air, there's obvious evidence of Katrina's wrath. Wind and waves have cut channels through once-uniform mats of grasses.
At the Chandeleur Islands, a crescent of land about 60 miles east of New Orleans, the full force of Katrina is more evident. Nothing's left but patches of marshland, or "island marshes." It will take years for the islands to recover -- but what did survive is held together by island marshes, a "green glue" that will anchor new growth.
In the marshes to the south of the city, the marshes have held up well. The storm wasn't a fatal blow, but scientists say that unless erosion is held in check, the marshes will continue to recede and leave New Orleans even more exposed to the elements.
But can the city remain a vital shipping destination, with all the deep-water channels required, and still divert enough Mississippi River sediment to the marsh to keep it alive?
"It's just a question of engineering and money," says the U.S. Park Service's David Muth, whose own home was flooded by five feet of water when the city's levees gave way. "The cheap way is the way we're doing it now -- so ask yourself the question, was that the smartest thing we could have done?
"You can rebuild Louisiana's marshes over the next 50 years -- it's just a matter of making the decision and doing it. And if this doesn't spur us to do it, we'll never do it."
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