The idea that there are usable nuclear bombs on the ocean floor unmonitored and ready to be picked up is still very frightening.
Would not the casing and the entire bomb itself be crushed by pressure?
That might depend-I'd think so if you are talking about ICBM's, but if you are talking about tactical nukes, as in torpedoes, they would need to be able to withstand as much pressure-or more-than the sub. Besides, uranium/plutonium are elemental...what are you going to crush them into?
Anyone who wanted fissionable material likely purchased it from eastern Europe when the USSR fell apart-as far as making the bomb, that's 70 year old technology...with enough money I'd suspect anyone could pull off a nuke. A delivery device might be harder to pull off clandestinely.
That said, I can't think of anyone I would be more fearful of having nukes than the lunatics that I KNOW have them now...