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If a glass bottom boat or a shallow "snorkeling"-style submarine has an accident 3 metre below sea level, chances are I can just swim to the surface and survive unhurt.

glass bottom boat
Source: viator.com

Maybe I can survive if such a boat crashes in shallow water at 10 metre depth (not sure).

If, on the other hand, a submarine implodes more than 3,000 metre below the surface, chances are that I die so quickly I don't even have time to know that I'm dying.

What's the deepest where a healthy, able-bodied human with normal swimming skills has a chance of surviving a submarine accident?

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  • This is going to depend on so many different factors. Like is the sub designed for easy egress (or bolted shut from the outside like the Titan), or did the accident happen slowly or fast (catastrophic failure can happen at any depth), or is the sub pressurized or not, what temperature is the surrounding water, and are you fighting off 100 other people who also want to escape. When you take away all these factors the question devolves into "how deep can I hold my breath and still get to the surface" which has nothing to do with submarines.
    – Peter M
    Jun 26, 2023 at 16:08
  • Escaping submarines on serious dept require you to exhale all the time going up, holding your breath will kill you.
    – Willeke
    Jun 26, 2023 at 17:22
  • Related: Barotrauma and Decompression sickness.
    – jcaron
    Jun 26, 2023 at 18:30

3 Answers 3

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That depends on so many different factors, that it's not really possible to give a quantitative answer. Some factors are

  • Specific manner of the submarine failure, e.g. Implosion vs. gradual flooding.
  • Manner of exiting the Sub.
  • physical fitness, swimming abilities
  • Any type of medical condition (this is the #1 cause of fatalities for scuba divers)
  • mental fitness (many scuba accidents are due to panic and disorientation).
  • training and preparation. Even having just a basic open water dive certification will increase your chances by a lot.
  • Environmental factors: day/night, water temperature, visibility, currents & waves, etc.
  • luck or lack thereof

As Weather Vane points out: the current record holder was Bill Morrison with about 60 m. And Bill did indeed get very lucky: The boat took on water gradually and hence compressed the air gradually. He was in the loo at the time of accident and got expelled in an air bubble when the hatch blew. Three other crew members were not so lucky and died during that incident.

There are is a non-trivial amount amount of scuba fatalities at depth of less than 10 meters, so no depth is truly "safe". Level 1 open water divers are certified to dive down to 18 meters. I'm guessing that even 10 meter will pose a significant risk for a person that has no particular risk factors, is reasonably fit but has no dive training or experience.

Things look much grimmer if the submarine suffers sudden implosion. The instant compression is likely to do a lot of damage and it might be difficult to get out.

The current world record for free diving is over 200 meter but this requires years of training and (in this specific category) access to assistive devices.

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  • is it even possible to go back up from 200m without assistance?
    – njzk2
    Jun 27, 2023 at 19:38
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Forbes reports

The deepest unassisted submarine escape on record was by British submariner Bill Morrison in 1945 from a submarine sunk in Loch Striven in Scotland. He made it out through an escape hatch from a depth of more than 200 feet. Morrison surfaced severe bleeding from his nose, ears, and mouth, and had pains in his head, neck and shoulders that persisted for years afterwards.

The biggest problem you might face escaping from a vessel with its hull intact, is to open a hatch.

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From 70 meter it will be critical

Assuming the submarine hasn't imploded rapidly, killing or maiming you through shock-wave or flying splinters, the biggest hazard will be the air you breathe.

70 meters is the depth from which both components of the compressed air (having density 8 times bigger than on surface) pose a grave danger for you. Nitrogen is narcotic, causing disorientation, nausea, panic attacks or even hallucinations (if you go deeper, coma and death can occur). Oxygen becomes toxic, and can cause uncontrolled cramps or paralysis, which can occur suddenly and without any warning. Both effects are stronger with the time you are breathing compressed air. With every second your tissues will become more saturated with nitrogen, which can cripple or even kill you in case you make it alive to the surface.

So you have really limited time to reach any opening, take your last breath and make your way towards surface. With the full lungs you should be positive, but you need to constantly breathe out so that your lungs won't explode because of air expansion. This is not the only danger, the way is long! Trained freedivers manage it to resurface with the speed of over 1 meter pro second, but they are well trained and have fins (though, they need to fight gravity because their lungs are more empty you can imagine on those depths). If you got panic, you might loose consciousness, breath out, and sink back to the bottom and die.

Considering you've reached the surface intact, the bubbles of nitrogen can form in your blood or nerve system, crippling you or killing you on spot, because your blood saturated with nitrogen in the depth, which can't find it way out quickly enough.

You'd have to be extremely lucky to survive ascent from more than 100 meters. Many scuba divers have died in that depth from nitrogen narcosis trying to break records with pressed air.

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  • Don't most submarines use uncompressed air for the passengers, relying on the strength of the hull to resist the water pressure? So I think air-pressure-related scuba hazards like nitrogen narcosis (and also oxygen toxicity, lung expansion injuries, decompression sickness etc) would only be an issue if someone put on scuba gear before they exited a failing sub, which seems unlikely. Jun 28, 2023 at 11:55
  • @user56reinstatemonica8 yes, but you need to equalise pressure in order to be able to open the dock and leave the submarine (the equalisation will happen anyway, but if it's too rapid, it will be explosive and will likely kill you on spot) Jun 28, 2023 at 15:20

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