Yeah, I had exactly the same thought! If it’s a single hull and not a double hull or something, and that’s the actual inside of the hull we are seeing, how the fuck is that safe? Someone uses too long of a screw and bow you have what, 400 atmospheres of pressure pushing against the presumably sharp point of the screw?

Strain monitoring built into the hull only helps you if you can surface in time once you notice a problem, and not at all if it is a catastrophic failure.

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.

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?

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.

Thanks for the rough numbers. And yes, that would be enough to kill people, but I probably overstated the effect on harder materials.

A pressure washer maxes out around 4000 PSI for a powerful one, and that can easily cut through flesh.

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?

Ah interesting - thank you for the explanation! That is just about as terrifying I think.

In this case the worry is implosion, not depressurization.

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.

There's a thread floating around on Reddit somewhere that said the implosive force had the same energy of 114 sticks of dynamite fwiw

Damn did you really need to add those graphic details. Enough internet for today.

The pressure is greater outside the sub. Nothing is getting sucked out.

Imagine someone falling onto that monitor at 4000 meters. When the operator accidently moves because of the joystick. If it wouldnt be so sad, it could be funny actually.

Also, why are they using carbon fiber and titanium? Genuine question - I would have thought that being light weight isn't a particular advantage for submersibles.

It's probably less about being light weight and more about high yield strength.