Under the cover of darkness in 1864, more than 100 Union soldiers broke out of Libby Prison in the heart of the Confederacy in Richmond, Va. A group of northern soldiers inside the prison found a way to dig a tunnel from the prison basement that let out beyond its walls. The basement was so infested with vermin they called it "Rat hell." But after a couple of weeks of burrowing, they surfaced inside a tobacco shed. One hundred and nine Union soldiers eventually escaped. Though many were recaptured and a few died in their attempt to make it back north, it remains the largest prison escape of the Civil War.