The photo shows that the bubble-like transparent window in the bow of the submersible is missing. I suspect that a structural failure began in the seam of the bubble, possibly causing a crack in the window that resulted in a catastrophic implosion within a few milliseconds.
12,500 ft/ 33 = 378.79 atmospheres, almost four times the surface pressure of Venus.
Mercifully, death would have been within a millisecond for the passengers.
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I agree, they didn’t feel a thing.
The window manufacturer would not guarantee the window’s integrity at the depth that they were using it, but they used it anyway........................