Meteorologists were able to warn people about the major storms known as Hurricane Katrina on August 23, 2005. It formed over the Bahamas and by August 28, evacuation took place. It was not a surprise that New Orleans was specifically at risk because in the past centuries, hurricanes have flooded New Orleans. Consequently, half of the city lies above sea level causing the aftermath of New Orleans to be completely surrounded by water. Although the Army Corps of Engineers had built a system of levees and seawalls, some were not as strong and sturdy as others. For instance, the levees along the MIssissippi River were strong and sturdy, but not the ones from Lake Pontchartrain, Lake Borgne and the water clogged swamps and marshes of New Orleans’ east and west. This caused greater risk to those who lived below sea level. Before Hurricane Katrina hit New Orleans, the mayor Ray Nagin, ordered an evacuation. This allowed people who didn't have a safe shelter to withstand the storm or could not leave the city immediately to stay in one of New Orleans’ stadium, the Superdome.