This is a crawfish chimney.
Under every chimney is a burrow which may extend down in the earth to 3 ft or more.
The crawfish use thier legs and mouth parts to dig up mud and make it into a pellet.
Each pellet is taken to the surface where the next pellet is placed beside the previous one.
The pellets continue to be stacked until the chimney is complete.
It is thought that chimneys are so that the crawfish never leave the burrow and
thus protect them against predators. It is also thought that the chimneys improve
air flow (oxygen) in the tunnels.
For more information on crawfish chimneys see:
http://www.americaswetlandresources.com/wildlife_ecology/plants_animals_ecology/animals/invertebrates/crawfish.html
Auburn, Alabama