I heard an anecdote at one point that when a submarine depressurizes that deep, everything inside gets pushed through whatever hole caused the depressurization, like squeezing the contents out of a tube of toothpaste.
"Hellevik, being exposed to the highest pressure gradient and in the process of moving to secure the inner door, was forced through the crescent-shaped opening measuring 60 centimetres (24 in) long created by the jammed interior trunk door. With the escaping air and pressure, it included bisection of his thoracoabdominal cavity, which resulted in fragmentation of his body, followed by expulsion of all of the internal organs of his chest and abdomen, except the trachea and a section of small intestine, and of the thoracic spine. These were projected some distance, one section being found 10 metres (30 ft) vertically above the exterior pressure door."
I wonder if anyone knows if this universally true? Would the inhabitants of this submarine have experienced a similar fate if one of those bolts failed or a hole in the hull developed?
That was a decompression event with high pressure air flowing out. If a submarine has a rupture that deep, it would go the other way, high pressure water coming in. Most likely, as soon as there was any break the whole thing would collapse in on itself. Picture yourself crushing an empty coke can between the two flat ends. It goes from intact and pretty strong to all of a sudden easy to push and crushed.
In the event you could contain it to a simple pinhole and not have it instantly crush, the stream of water would cut anything placed in its path. It would work similar to a lightsaber.
Keeping it to round numbers, pressure at 4000m is about 400atm, or a bit under 6000 PSI. From what I can see, a waterjet cutter ranges from 20,000 PSI to 100,000, depending on model and which source you trust. So if they could contain it to a pinhole, they'd probably be lucky enough that the incoming stream wouldn't cut through the far side, though the same isn't true of the occupants.
Though as you said, the far more likely outcome is that the vessel would crush almost instantly.
Also, at that depth, the crushing pressure of an implosion would instantly superheat the air within the hull like a diesel engine combustion chamber. Which would win - flashed into ash or compressed into goo?
No — that incident was due to a high pressure area (saturation diver living quarters) being opened to a low pressure area (the atmosphere at large), resulting in explosive decompression. The submarine in question is a low pressure area in a high pressure environment, so essentially the inverse of the Dolphin accident.
Might have been from the Byford Dolphin accident: https://en.wikipedia.org/wiki/Byford_Dolphin
"Hellevik, being exposed to the highest pressure gradient and in the process of moving to secure the inner door, was forced through the crescent-shaped opening measuring 60 centimetres (24 in) long created by the jammed interior trunk door. With the escaping air and pressure, it included bisection of his thoracoabdominal cavity, which resulted in fragmentation of his body, followed by expulsion of all of the internal organs of his chest and abdomen, except the trachea and a section of small intestine, and of the thoracic spine. These were projected some distance, one section being found 10 metres (30 ft) vertically above the exterior pressure door."
I wonder if anyone knows if this universally true? Would the inhabitants of this submarine have experienced a similar fate if one of those bolts failed or a hole in the hull developed?