What does the 'U' in U-boat stand for and how many of them are left?

Few naval vessels have developed as enduring of a legacy as German U-Boats, but what does that name actually mean?
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This is how men lived and died - inside U-boats

Life aboard a German U-boat in World War II was brutal, claustrophobic, and deadly. Follow Hans Fischer, a young mechanic turned U-boat crewman, as he endures stifling heat, toxic diesel fumes, no ...