so as a DIY submersible fan I've been following this titan thing more than the usual newsbite of the day.

the likeliest scenario is catastrophic hull failure due to fatigue in the carbon fibre, in which case the mighty pressure of the depths would have crushed them instantly

if that's the case, the searchers would probably have been informed by the US Navy of a loud bang showing up on sonar surveillance at the approximate time and place of the dive, and all the while they're just trying to find the wreckage bc they know such a scenario would've been immediately fatal

second most likely scenario (which has apparently happened before) is some sort of severe mechanical/electronic failure, which at the least terrifying would leave them on the surface, unable to call for help, stuck in a sealed carbon fibre tube and at the worst would mean drifting several km down

the distal cause of all this is the complete disregard of competent engineering practices and common sense. the risks of using carbon fibre and non-spherical pressure vessels in such applications is well known, and the additional complexity of the end caps didn't help. not getting classed is one thing, but the sheer amount of disregard for reasonable procedures and design (bolting the hatch to the hull given the need for speedy egress stands out) more or less made something like this inevitable