A group of three men and a woman were "exploring" the US Steel property at 941 Collins Street around 6:47 p.m.
They entered the pump house and found a body floating in four to five feet of water, Joliet police said.
The group immediately called 911.
Joliet Fire officials removed the body of a man, which was "in an advanced stage of decomposition."
The man's identity and cause of death have not been released.
Anyone with information is asked to contact police.