Very sophisticated inertial guidance systems. Combined with occasional satnav validations.
During WW2 the US Navy built 126 submarines.
1. Ballistic submarines 2. Attack submarines
No sorry
75
52 US submarines were lost in WW2. This was about 1 in every 5 submarines that the US had in WW2
Yes, submarines were used by the Confederate Navy in the US Civil War.
The US Navy currently has 14 active duty Trident (Ohio Class) Ballistic Missile Submarines. They are scheduled for replacement beginning in 2028.
Submarines and Aircraft Carriers
$2.4 billion for the newest Virginia Class attack submarines
Sonar
By themselves - they can't. But these days we have machines and contraptions that help us with that. Diving gear, diving bells, submarines etc all allow people to breathe underwater.
Underwater Demolitions Training. US Navy S.E.A.L. basic training. Usually goes with B.U.D.s - Basic Underwater Demolitions.