The Flint River, which starts just North of the airport, actually winds its way underneath the airport.
The river isn't what you would expect. You'll find most of it in ditches along roadways and flowing through pipes under Atlanta. The river tunnels under the airport, eventually meets up with the Chattahoochee River at the Florida border, then goes on to form the Apalachicola River and empties into the Gulf of Mexico.
An organization dedicated to restoring and revitalizing the river, called Finding the Flint, is funded by American Rivers, The Conservation Fund, and the Atlanta Regional Commission.