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Meet William Jackson.  He was a slave of Confederate president Jefferson Davis — and a spy for the Union army.  It turns out that a number of slaves engaged in espionage for the north.  Collecting information wasn’t hard for them; because of their menial status, they were simply ignored as Confederate leaders openly discussed war strategy in their presence.  Because of their tradition of oral history (and reliance on memory since teaching them to read and write was illegal), the slaves were able to remember even minute details.

Another interesting note: In addition to her Underground Railroad activities, Harriet Tubman also led a number of espionage missions, and even an 1863 raid in South Carolina.