In 1864, a combat submarine sank an enemy warship for the first time ever - and it was a Confederate submarine. I was not aware that the Confederacy even had submarines, let alone an extremely important one. (To be fair, I don't know much about submarines in general.) The submarine, the H.L. Hunley, sank herself while carrying out her mission. No living souls knew why. The Hunley just disappeared. P.T. Barnum put up a $100,000 reward after the Civil War to anyone who could find her, but the Hunley was gone.
Gone, that is, until 1995, when Clive Cussler - yes, the NYT bestselling author - discovered the wreckage at the bottom of Charleston Harbor. She was brought above water, and now you can visit her in North Charleston (but only on weekends).
(h/t to Brian)