the oldest cemeteries here are below ground. There isnt any real reason anymore other than tradition for above ground crypts; concrete sarcophagi arent affected by water.
So, Metairie Cemetery and all the tombs are just for show?
Not completely true.
If the sarcophagus is properly sealed the ground water will eventually push it to the surface. The sarcophagus will literally float to the surface.
It may take a few decades but given that New Orleans is river delta land and has high ground water probably not too long.