South Yorkshire Police officers were called to High Melton College at about 14:45 GMT on Tuesday where workmen had found what appeared to be a body.
Dark and cold conditions prevented officers from accessing the body so the scene was guarded overnight.
Detectives are working with Yorkshire Water, underwater search teams and crime scene officers to determine the circumstances surrounding the incident.
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High Melton is a campus of Doncaster College.
A body has been found in a sewage tank at a Doncaster college.