On a cold February night in 1864, the Confederate submarine CSS H.L. Hunley sank a Union ship and then sank into the waters off South Carolina, never to be seen again. The Hunley was finally located and brought to the surface 136 years after it sank. Witness the raising of one of the world's first submarines as National Geographic investigates the final moments of its doomed voyage and the mystery surrounding the disappearance of the South's secret weapon. What really happened to the Hunley and her crew? Why did she sink? What artifacts are still on board? Find out as we journey deep underwater for answers.