Confederate Land Mines
Land mines or torpedoes buried along the western approaches to the fort caused most of the casualties to the Union troops in the assault on December 13, 1864. After the taking of the fort, General Sherman personally gave orders that the captured Confederate garrison be required to remove the unexploded mines.
Georgia Historical Commission 1963
Plaque courtesy Lat34North.com.
Original page, with additional info, here.
Photo credit: Byron Hooks of Lat34North.com.