A British explorer has found a submarine in Panama that dates back to 1864. It’s shape and the use of a lock-out system for divers to enter and exit underwater suggest it inspired the design of the Nautilus in Jules Verne’s 20,000 Leagues Under the Sea, published five years later.
The Explorer was abandoned after its crew died of a “fever” which might actually have been the bends. It was built out of cast iron by an inventor called Julius Kroehl for the Union forces in the US Civil War, but never used. I need to do some research and see how many other Civil War subs there were. The most famous was the Hunley, the first sub to sink a ship, but there must have been others.