They are very real.
The marines do not use submarines. Only the Navy does.
german submarines
german submarines
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Submarines are used both for military and rersearch purposes.
military submarines use low frequency antenna that extends from the sail(conning tower.) The message is typed into a computer, then is sent to another submarines antenna that receives it and displays it on a computer.
the military and navy use it for war and combat. Many submarines are made for launching missels and taking down ships and other subs.
Civilian-Under water research. Military-Stealth
Francis C. Malko
As regards military submarines meant as warships, the Russians built the submarines with the deepest diving capability, but U.S. submarines generally make less noise.
No - most military submarines are limited to crush depths above 2,000 feet.