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(CNN) —  

New Orleans was built above sea level, but over time, it’s been sinking.

And from Hurricane Katrina in 2005 to threats of flooding this week, a few facts on – and in – the ground explain why the Big Easy is uniquely vulnerable to massive flooding.

1. When it was built, it was barely above sea level

The original part of the city, the French Quarter, was built on higher ground beginning in the early 18th century.

Settlers who got the best land were able to build only about 10 feet above sea level. Even from the beginning, the city was fighting an uphill battle as it expanded. New Orleans is mostly flat, and areas around the French Quarter are just a little lower.

But in this situation, every foot counts.

“Even during its very beginnings, New Orleans’ residents understood the value of land elevated above the flood-prone land they had chosen to call home,” the US Federal Emergency Management Agency says in its History of Building Elevation in New Orleans.

2. It was built on loose soil

As the city grew, architects chose to build shorter houses and structures, out of fear that the ground couldn’t support anything taller.

“Though a few [structures] climbed as high as three and sometimes four floors, most hovered around two or two-and-a-half stories, since builders feared that the town’s spongy soil couldn’t bear the added weight,” New Orleans historian Lawrence N. Powell wrote in “The Accidental City: Improvising New Orleans.”

Floodwaters creep up the wheels of a parked car in New Orleans on Wednesday, July 10.
PHOTO: Matthew Hinton/AP
Floodwaters creep up the wheels of a parked car in New Orleans on Wednesday, July 10.

3. A drainage system had unintended consequences

Later, a “sophisticated municipal drainage system installed around 1900 allowed the city to spread onto former marshes but also starved the land of replenishing sediment” and removed water from the soil, explains geographer Rich Campanella, a professor in Tulane University’s School of Architecture.

Without sediment and water to stabilize the ground, the “former marshes sunk as much as 8-12 feet,” and wetlands rapidly eroded, Campanella wrote in a study.

By the 1930s, one-third of the city was below sea level, according to the New Orleans Times-Picayune. And by the time Katrina struck, that number was up to about 50%.

4. Sea levels are rising

Much of the area around New Orleans is now 1½ to 3 meters (4.92 to 9.84 feet) below mean sea level, according to a 2003 study by the US Geological Survey. Scientists found that the ground in the area was sinking at a rate of 1 centimeter a year.

That continual sinkage, combined with rising global sea levels due to the climate crisis, meant New Orleans would probably be between 2½ and 4 meters (8.2 to 13.12 feet) below sea level by 2100.

Correction: A previous version of this story had incorrect conversions of meters to feet.