While people often tend to think of submarines as a modern invention, inventors created several prototypes from 1580 onward, and they even attempted to use them. Some of the designs remained theoretical and never got off the drawing board, but as time went on, several Europeans built and tested primitive submarines. Most of these early subs were made of wood and often possessed an awkward shape, but a French priest first proposed the now familiar cylindrical layout in 1634. He determined that this shape would make it easier for the submarine to withstand water pressure, while likewise improving maneuverability.

The American Civil War witnessed new designs and prototypes of submarines appearing on American naval drawing boards. These subs had the “cigar shape” familiar throughout history from that point forward. Most of them failed for one reason or another, but one, the CSS Hunley, actually succeeded in using a “spar torpedo” to sink the USS Housatonic. It was the first time a submarine successfully sank a ship, but as historic and groundbreaking as the Hunley and its mission was, all of that was overshadowed by the lingering debate over just what happened to the Confederate submarine, which never made it back to port after the attack.

Although the Hunley remains the most famous submarine of the Civil War, the Union Navy attempted to create and use a submarine years before the Hunley embarked on its historic operation, and though it’s widely forgotten today, the first Union submarine, the Alligator, also had a fascinating history.