Very interesting, guys. This should give me some interesting reading to do. I already read about the USS Tang, it's a good experiance to read, especially with the prison parts afterward. I had heard how the Japanese Navy did not focus on protecting merchants (the same reason how they did not use their submarines to attack merchants).
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CptChacal I always thought it was the torpedo loading hatch too, but then how do you explain there's not one for the front tubes and also, this picture looks the same like the part of the submarine
http://www.navy.mil/view_single.asp?id=8910