"In a modern submarine, where crush depth is likely to be deeper than 1,000/2,000 feet, the hull is likely to implode, collapsing completely. In the process, the air inside the hull is compressed very rapidly, raising the internal temperature by several hundred degrees. Everything flammable inside, including the crew, is incinerated in the fraction of a second before the water rushes in to crush what's left. It is, indeed, believed to be very fast, probably faster than anyone aboard would be able to comprehend. You would simply and instantly go from being alive and wondering when it was going to happen to being dead and not knowing it had."

"With the rapid compression of the air, there would be a simultaneous step rise in both pressure and temperature. The air pressure would rapidly approach, but not exceed, the water pressure. The temperature would, of course, be proportional to the air pressure, but what the actual numbers are I am not certain. I am not sure one way or the other whether they would raise to the point of combustion, but I agree that it’s possible. In any case, they would be high enough to cause severe burning of the exposed body surfaces. Of course, the body would also experience the effects of the sudden high pressurization, resulting in burst eardrums, a ruptured body, perhaps cracking of the skull (which under a pressure increase of the rapidity would be like it’s own pressure-tight space, just as would the main body), the eyes would probably burst (implode), and there would probably be bursting of many blood vessels. Of course, both any burning effect, already mentioned, and the result of a rapid pressure impulse on the body would both be in a fractional second. Loss of consciousness and death would be near instantaneous, as you state. I can see perhaps enough of a window for the body and conscious to experience perhaps a very brief moment of thought and sensation, but if there is any at all, it would be exceedingly brief.

Given what we know from actual hull crushes (and I’m sure that someone knows waaaaay more than I do since I’m just going by what we learned as part of being nuclear submariners), the entire submarine would not necessarily crush all at once. One example is the Golf-class submarine that we recovered from the Pacific floor as part of our espionage operations (Howard Hughes’ Glomar Explorer, etc.) Though we only recovered about the front third of that submariner, we obtained about two thirds of the interior works, since the submarine breached at some point in the rear and the pressure surge slammed the interior forward. A second example is the U.S.S. Scorpion, our nuclear SSN which was lost in the Atlantic in 1967. We believe that we captured on sound detection devices the sequential collapse of pressure-tight bulkheads as it crushed, indicating that the hull breached to sea pressure at some point (certainly the bow compartment based on the cause for sinking) and then as the boat sank and the rising sea pressure exceeded the strength of the bulkheads they collapsed in turn from forward to aft. Of course, in this case, each compartment is acting as its own pressure-tight space. In both submarine cases, of course, the indication is that the whole sub did not crush all at once, but experience hull failure at one point, and then had a pressure wave travel inside the boat. (While it’s horrifying to consider, there is a good likelihood that the men in the rear-most compartment of the Scorpion heard the collapsing of the forward bulkheads before the engine room bulkhead failed.) Given the experience of the Soviet sub, we also know that the forces are strong enough to rip machinery from it’s mounts and move it, so this adds the final event on the bodies inside the sub, as they are mutilated in varying ways as they are caught in both the movement and crunch of machinery and/or collapsing bulkheads or the hull."

"
Theoretically, temperature rise during compression is given by:

T2/T1=(P2/P1)^((n-1)/n)) where n is about 1.3 for air.

If we start at 70F, T1= 530 R (degrees Rankine).
P1 = 1 atmosphere
P2 = 2000 ft water = 867 psi = 59.4 atm

We have:

T2/530 = (59.4)^(1.3-1)/1.3)

T2 = 530(59.4)^(0.231)
T2 = 5302.57 = 1362 deg R = 901 deg F

Hot indeed!"

http://www.fleetsubmarine.com/phorum/read.php?f=5&i=20&#...

"I've heard speculation that the implosion would be similar to what happens when you set off a bomb calorimeter, in which the extreme pressure rise would heat up the internal atmosphere to the point that combustible materials would be incinerated as the water rushed in.

For illustration purposes, it can be calculated that a hole in the submarine's hull of just 1-foot diameter (at a depth of 800 feet) would fill the associated compartment in just a few seconds. Larger hull ruptures would flood the submarine essentially instantaneously."

http://boards.straightdope.com/sdmb/showthread.php?t=553491

"The myth that I heard a number of times when I was in the Navy was that the air in the sub will actually detonate due to the sudden increase in pressure which causes it to heat rapidly, and the crew will die from the ensuing firestorm long before they get a chance to die from drowning or the pressure."

"The "long before" comment was considering that all the things that are going to happen to you in this scenario are going to happen pretty quick. But the implosion will be ship-wide since the hull is likely to give way in more than one location and, LA class subs at least, effectively have only one compartment with people inside (there are actually 2 but the door is usually open). The pressure will be felt throughout the ship at the same time and therefore the fire will happen ship-wide as well."

http://community.discovery.com/eve/forums/a/tpc/f/9701967776...