The Sub, had gone down 25 times!! the useful life of a sub like this going that deep is 2 maybe 3 dives and it is scrapped. The Hull can not handle the compression decompression cycles, and every cycle stresses the hull even more, until???
There was engineer that worked there and when he brought this they Fired Him.
That's a lot of stress fatigue! Aiplanes undergo rigorous examination every so often as laid out in FAA guidelines. I don't know if the sub was subject to any such. I read today that although the passengers had signed waivers the company itself can still be sued for criminal negligence.