Sometime before 9:50 p.m., a car plunged into the Midlothian Creek Detention Reservoir at 167th Street and Cicero Avenue, Oak Forest police said.
A good Samaritan pulled the driver out of the vehicle and helped the individual to the shore.
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That driver was taken to the hospital to be checked out.
South suburban emergency responders were seen working to pull the car out of the water later Thursday night.
It was not immediately clear how the driver ended up in the water.