New Orleans was originally settled on the river's natural levees or high ground. After the Flood Control Act of 1965, the US Army Corps of Engineers built floodwalls and man-made levees around a much larger geographic footprint that included previous marshland and swamp. Over time, pumping of water from marshland allowed for development into lower elevation areas. Today, half of the city is at or below local mean sea level, while the other half is slightly above sea level. Evidence suggests that portions of the city may be dropping in elevation due to subsidence.