Katrina was terrible, but it's not the worst storm in the Gulf Coast. The Galveston Hurricane of 1900 took an estimated 6,000 to 12,000 lives, mostly in Texas, in September 1900 and is considered the deadliest hurricane in U.S. history.
The storm didn't become a hurricane until passing west of the Florida Keys where a sharp left turn sent it heading straight toward Galveston. That gave residents and local officials less than four days to prepare. The Category 4 storm brought 20-foot (6-meter) storm surges and flash flooding and even pounded Oklahoma and Kansas when it was done with Texas. More than 3,600 homes, as well as a number of structures believed to be "storm proof" were destroyed in the hurricane, whose damage totaled $30 million [sources: Ramos, The Weather Channel].
Galveston took some amazing steps to ensure the damage was not repeated. It built a 3.5 mile, or 5 kilometer, seawall (later extended to 10 miles or 16 kilometers) and raised the level of the entire city, in some places as much as 16 feet (5 meters) [source: Ramos].