Yes, they are real.
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.
Civilian-Under water research. Military-Stealth
the military and navy use it for war and combat. Many submarines are made for launching missels and taking down ships and other subs.
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.