By the time Hurricane Katrina made landfall near Buras, Louisiana early on the morning of August 29, 2005, the flooding had already begun.

At 5 a.m., an hour before the storm struck land, the U.S. Army Corps of Engineers, which administers the system of levees and floodwalls in and around New Orleans, received a report that the levees of the 17th Street Canal, the city’s largest drainage canal, had been breached. East of the city, massive storm surges sent torrents of water over the levees along the Mississippi River-Gulf Outlet (MRGO) and into St. Bernard Parish, located just southeast of New Orleans.

In all, levees and floodwalls in New Orleans and surrounding areas fell in more than 50 locations during Hurricane Katrina, flooding 80 percent of the city and fully 95 percent of St. Bernard Parish.

Though thousands of New Orleanians evacuated in the days leading up to Katrina, around 100,000 people remained in the city. Flooding caused power outages and transportation failures throughout the city, making the emergency response to the storm even more difficult. In particularly hard-hit areas, like the Lower Ninth Ward, the water reached depths of up to 15 feet, trapping many people in houses on roofs or in attics for days before they were rescued.

The exact death toll is still uncertain, but it’s estimated that more than 1,500 people in Louisiana lost their lives due to Hurricane Katrina, many of them due to drowning. The devastation caused by the storm, and the accompanying failure of the levees, left millions homeless in New Orleans and along the Gulf Coast, and some 400,000 residents ended up leaving the city permanently.

In the immediate aftermath of Katrina, federal officials—including Michael Brown, director of the Federal Emergency Management Agency (FEMA), who later resigned over his handling of the Katrina response, and President George W. Bush—claimed that the catastrophic failure of the levees in New Orleans was something that no one could have foreseen. “I don’t think anybody anticipated the breach of the levees,” Bush said on September 1, 2005, during an interview with Good Morning America.

But the levee failures weren’t a complete surprise. For years before Hurricane Katrina, scientists, journalists and emergency officials had been worrying about what could happen if a major hurricane were to hit New Orleans.

During Hurricane Georges, a Category 2 storm in 1998, waves on Lake Pontchartrain, north of the city, had reached within a foot of the top of the levees, reported John McQuaid and Mark Schleifstein in the New Orleans Times-Picayune in 2002. “A stronger storm on a slightly different course...could have realized emergency officials' worst-case scenario: hundreds of billions of gallons of lake water pouring over the levees into an area averaging 5 feet below sea level with no natural means of drainage,” they wrote, three years before Katrina hit.

Fears about flooding go all the way back to the founding of New Orleans on land in 1717, by the French-Canadian explorer Jean-Baptiste Le Moyne, Sieur de Bienville. Human intervention—including expansion onto drained swamplands surrounding the original city—and the erosion of coastal wetlands only made things worse over the centuries. By the time Katrina arrived, New Orleans lay at an average of six feet below sea level, with some neighborhoods even lower than that.

Surrounded by water—Lake Pontchartrain to the north, and the Mississippi River to the south—and bordered by swampland on two sides, New Orleans has long relied on a system of levees to protect it from flooding. But the city’s low elevation, and its position within the different levee systems, creates a so-called “bowl effect,” meaning that when water gets into the city, it is very difficult to get it out. During Katrina, with many pump stations damaged by the storm, the water stayed in the bowl.

Before Hurricane Katrina, New Orleans hadn’t experienced a major hurricane for 40 years. After Hurricane Betsy flooded the city in 1965, killing several dozen people and causing more than $1 billion in damage, Congress authorized the U.S. Army Corps of Engineers to begin a major overhaul of the region’s hurricane protection system. Yet due to budget cuts and various delays, the project was only 60-90 percent complete by the time Katrina hit, according to a report by the United States Government Accountability Office.

In the immediate aftermath of Hurricane Katrina, the U.S. Army Corps of Engineers claimed the massive storm had overwhelmed the levee system, which had been designed to protect the region from a Category 3 storm or below. Yet later investigations revealed that some of the city’s levees failed even at water levels far below what they had been built to withstand.

In June 2006, the Army Corps issued a report of more than 6,000 pages, in which it took at least some responsibility for the flooding that occurred during Katrina, admitting that the levees failed due to flawed and outdated engineering practices used to build them. Yet debate continued over where blame lay for the disaster: The report also called out local officials for pushing the Corps to build the less-effective hurricane protection system, claims that the report’s lead author later concluded were not justified, according to a 2015 report in the New York Times.

Over the decade following Hurricane Katrina, federal, state and local governments spent more than $20 billion on the construction of 350 miles of new levees, flood walls and other structures. The improved system is designed to protect New Orleans from storms that would cause a so-called “100-year” flood, or a flood that has a 1 percent chance of occurring in a given year.

Even with this vast expenditure, experts continue to question whether New Orleans is truly safe from the next big storm.