A group of three men and a woman were "exploring" the US Steel property at 941 Collins Street around 6:47 p.m. on Tuesday.
While they were inside the pump house, they found a body floating in four to five feet of water, according to Joliet police.
The group immediately called 911.
Joliet Fire officials removed the body of a man, which was "in an advanced stage of decomposition."
The man was later identified as Warren Baker. A 44-year-old man who had been reported missing on May 13.
According to police, Baker was unhoused and was last seen in the 400-block of East Cass Street on April 28.
Baker's cause of death has not been released.
Anyone with information is asked to contact police.