Built as a stealth weapon long before the term existed, the sleek submarine “H.L. Hunley” was the pride of the Confederacy, the first sub ever to sink an enemy ship in combat. But after its only mission, the secret weapon vanished with its crew off the coast of South Carolina. The lost sub lay hidden for more than a century until 2000, when it was recovered and placed in a state-of-the-art conservation laboratory. Only now are scientists uncovering the truth behind its disappearance.