In the 20th century, New Orleans' government and business leaders believed they needed to drain and develop outlying areas to provide for the city's expansion. The most ambitious development during this period was a drainage plan devised by engineer and inventor A. Baldwin Wood, designed to break the surrounding swamp's stranglehold on the city's geographic expansion. Until then, urban development in New Orleans was largely limited to higher ground along the natural river levees and bayous. https://en.wikipedia.org/wiki/A._Baldwin_Wood

Wood's pump system allowed the city to drain huge tracts of swamp and marshland and expand into low-lying areas. Over the 20th century, rapid subsidence, both natural and human-induced, resulted in these newly populated areas subsiding to several feet below sea level.