Submarine escape

 "No man is more hated than he who speaks the truth." - Plato

 Submarines are equipped with a means of exiting the ship while stricken on the ocean floor.  It's possible that sort of emergency could arise, that the ship would be unable to surface on its own.  This happened to at least three submarines: K-141 Kursk, and SS-192 Squalus (re-named "Sailfish"), and SS-306 Tang (sunk by her own torpedo).

Below:  The means of exiting a stricken submarine is called the "escape trunk".  This image is of a submarine used for public tours, thus the "Exit" sign.  The ship's crew are well aware where the exits are.

How the escape trunk works is this:  There is an upper and lower hatch, with a compartment rated for full test depth seawater pressure.  A small group of sailors enter the compartment, close the lower hatch, and fill the compartment most of the way full with water.  Then high pressure air is admitted to equalize the compartment with outside seawater pressure, and the upper hatch is opened.  The sailors then exit the escape trunk and perform a "free ascent" to the surface.

Below is a trainer for that sort of free ascent.  It's inside a tower containing a column of water about 50ft deep.   

Below is the submarine escape training tower in Pearl Harbor, Hawai'i.

I never attended submarine training school, so I never had to complete submarine escape training.  I'm pretty sure that I'd have passed the course.  One thing I can assure you of though, is that having to actually use the escape trunk and perform a free ascent in a life or death situation would be PTSD-inducing.  

Imagine having to fill the compartment up with cold seawater to your neck, leaving a foot of air above you to breathe - and hoping things went right and it didn't fill completely up with water, drowning you and your buddies.  Next would come the equalization with external seawater pressure.  This has to be done quickly, because the longer your body is at depth, the more nitrogen accumulates in your bloodstream, and the more likely you are to get decompression illness ("the bends") when you reach the surface.  So the air blasts in deafeningly at up to 450 psig, possibly rupturing your eardrums due to the rapid pressure change.  Next you open the hatch, completely flooding the compartment, and lose your remaining breathing air.  

Now you do a free ascent to the surface (without any environmental suit - we didn't have space to carry those on board), and you yell "HO HO HO HO" all the way to the surface, to release the air in your lungs as you rapidly and uncontrollably decompress.  If you hold your breath on the ascent, that air will expand and rupture your lungs.  Fun stuff.

There is a limit on how deep you can perform a free ascent, and that limit is based on your own body.  You *will* get the bends performing a free ascent below about 600ft.  The human body will absorb nitrogen quickly enough at pressure that upon decompressing, your blood will foam like a fizzy soft drink - this is the physical nature of decompression sickness.

Here's the thing though - most of the ocean is quite a bit deeper than the crush depth of a submarine hull, and very little of the ocean is less than 600 feet deep.  In order to have any chance of survival at all, a ship would have to settle to the bottom on the shallowest part of a continental shelf or in a shallow sea.  If I remember the table correctly, even at 450 feet, you have only about 90 seconds to pressurize, escape and begin the ascent to start decompressing.  A deep submergence rescue vehicle would definitely be preferable!

 Whenever we came into port, I often had to open the upper hatch of the engine room escape trunk.  This had to be done in order to connect shore power, so that the reactor could be shut down.  There is a void space between the deck of the submarine and the actual hatch sealing area that contains quite a bit of water.  There's a drain, but that would usually be plugged.   

The guy who got to open the upper hatch would close the lower hatch - so that gobs of residual seawater didn't soak the engine room floor.  Then he would crack the upper hatch and let 30 gallons or so of residual seawater pour into and start filling this cramped compartment.  I remember thinking to myself one time, "How many people could do this without freaking the hell out?"  I'm guessing not too many.