As others have noted, crushing is an unlikely cause of death for a diver. Even recreational divers experience pressures up to 4 to 5 times atmospheric. Physically, you do not even sense the difference. Water, and the human body does not compress significantly under pressure, and a diving regulator ensures that the gas in your lungs/ears etc. are at the same pressure as the depth to which you are diving. Problems come with phenomena related to gas solubility. Even recreational divers need to be familiar with the symptoms of a condition known as nitrogen narcosis. Initially this produces a sensation akin to getting a bit light headed, and is caused by high levels of dissolved nitogen in the blood. In extreme cases it will result in unconciousness and death. Nitrogen narcosis can occur at depths as shallow as 40m. Extreme divers use special gas mixtures to avoid nitrogen narcosis. By replacing nitrogen with helium, divers can go deeper. Ultimately, however, oxygen itself becomes toxic. Oxygen poisoning will cause unconciousness and death.
Also note that the OP is asking about the extreme depths of the ocean. One mile down isn’t even the average depth of the ocean.
At a depth of approximately 10,000 m, pressures will reach approximately 1,000 atmospheres (1 x 10[sup]8[/sup] N m[sup]-2[/sup]). That’s 14,700 pounds per square inch. I would imagine that the pressure you apply to your blood vessels to cut off circulation when your foot goes to sleep is considerably less than 14,700 psi.
I really do think your circulatory system would fail dramatically at that depth. Come to think of it, I don’t think there will be enough of your skin or bones intact for you to care, either. 
But why should that happen, assuming there are no gas bubbles to compress? Fluids and solids are relatively uncompressable, right? And it isn’t like the water is pushing against just one part of you like when you apply pressure to a specific part of your body and it makes something fall asleep[sup]1[/sup] – if you were to suddenly apply 14,700PSI to your thigh, then yeah, I can see how it would be devastating to the underlying circulatory system, as well as the underlying (everything else). But what about when you apply it all over, evenly?
[sup]1[/sup]On another note, as Cecil once wrote, when you make your foot fall asleep, it’s not because you’re compressing blood vessels and cutting off circulation – you’re compressing a nerve. If you slept on your leg funny all night and you actually were preventing blood from circulating normally, your leg wouldn’t be asleep in the morning, it would be dead, I’m pretty sure.
If the water contains dissolved gases (such as methane), freezing temp and behaviour is different, and deposits of Methane Clathrate can form - an icy mass containing methane on the bottom of the ocean. You can get the clathrate (it looks like ice) and burn it.
These appear to be stable, but methane releases from clathrate deposits could be implicated in unexplained ship disappearances - if the clathrate releases a large amount of methane round a ship, the water density drops, and so does the ship - to the bottom.
Catastrophic clathrate releases have been implicated in mass extinctions, and are currently being investigated as a source of hydrocarbons for when the oil runs out. The main issue is maintaining the stability of the body of the deposit while extracting the methane for the drilling area - not to mention 300m of water, just above freezing water, oceanic platforms etc.
Simon
That’s exactly it.
The relatively part of the “relatively incompressable” is that under temperatures and pressures likely to be encountered on the surface of the Earth, it’s easier to treat water as an incompressible fluid and the error introduced into your calculations by doing so is trivial. If you do so at the bottom of the ocean, where pressures differ form those at the surface by three orders of magnitude, you are ignoring the fact that water is a compressible fluid and are introducing major errors into your calculations.
As far as an “incompressible solid,” I’ve never heard of such a thing. Perhaps they exist inside neutron stars?
I suspect that when you apply that pressure over your entire body equally, (1) your heart is no longer able to force blood to move through your circulatory system by creating a higher pressure inside any of its chambers than exists in the blood vessels outside that chamber. (At least, any difference in pressure it could create is insignificant relative to the distances blood must be moved around the body.) (2) Every other major body system has failed, so it doesn’t really matter what your circulatory system is doing anyway.
Oops. My bad. Thanks for catching my mistake :), but I think you can still cut off blood vessels by applying pressure; perhaps the “foot asleep” was just not an example of that.
According to the link posted previously, at your example of 10,000m and 1,000 atmospheres of pressure, water compresses about 4.34%. Is that significant enough to do serious damage to your body?
Obviously, that kind of pressure can’t be good for you. But it’s less than a 5% change; how bad is that really? I’m not saying you’re wrong about the whole thing, but so far it seems to be just your opinion. Are there any cites available?
I mean, fish and cetaceans can survive intense pressure differences, and obviously their blood never stops flowing. I know their physiology is modified, but is it really that much different, such that pressure that would kill us (even if our air spaces were filled with some kind of oxygenated fluid) leaves them unscathed?
Hmmm. I just found this page which claims that tests have been done on mice using liquid breathing systems at pressures equivalent to 8000 feet, which is, according to my rough calculations, about 250atm. So obviously pressures that seem lethally high are not, when you are relatively incompressible. Now, obviously, the 1000atm pressure of your 10,000m depth example is four times higher, but the real question is, when does it become lethally significant?
I don’t think the problem here is how much the water is condensing… I think it would have more to do with the fact that you have thousands and thousands of kgs of pressure being exerted on you by the water above you at that point. Wouldn’t that be akin to going higher and higher in the atmosphere, and you eventually die due to lower pressure (Ok, so that would be lack of oxygen at the lower pressure.) I know that I wouldn’t want to sit in a tank with 22k kgs of mass on top of me.
True enough. It is only my opinion, mostly because I don’t know of any relevant cites, but am skeptical that things will be hunky-dory at such extreme pressures ;).
This site (and cite?) mentions tests with mice, but doesn’t seem mention under what pressures they were performed. I tried a google search for “liquivent mouse test pressure”, but my quick perusal of the results didn’t turn up high pressure tests. The Liquivent page itself does not seem to mention high pressure tests. Most of the concern about Liquivent seems to be around ensuring that organisms can survive replacement of air with Liquivent and its use in the treatment of lung injuries and respiratory distress.
Sadly, the science of surviving such high pressures appears to be very limited. You can’t tell me researchers are squeamish about imploding a few mice to find the truth! I wonder what the real reason is…
If true, the claims of mice surviving pressures equivalent to 8000 ft depths in the ocean almost certainly would show that the opinions I’ve expressed above in this thread are completely wrong. Now, can we find out if these high pressure tests have in fact been done? Or is it merely a rumor? However, keep in mind that the bullet point which says these tests have been done begins with the phrase “According to the movie ‘The Abyss’,”… there doesn’t seem to be a cite for the 8000 ft mouse tests.
There are no known cetaceans that can dive that deep (although many of them do go through large pressure changes during dives much shallower than the deep ocean trenches), and bringing live samples up from the deep ocean is extremely problematic. Lots of them die in the act of trying to bring them to the surface. Perhaps the reverse is also true were we to attempt to bring a live human to their pressure. After all, one thing to think about is that your blood is not just water; you also have red blood cells. Many of your capillaries are just barely wide enough to allow red blood cells through… what if those capillaries were squeezed tighter? Perhaps the red blood cells themselves would compress, but could their cell membranes and cytoskeletons withstand such high pressures? I suspect they won’t, because it seems unlikely that they would have sufficient extra strength to accomodate a 1,000 times more pressure than they or their ancestors had to face. On the other hand, I have no firm evidence that they would not survive such a pressure. An experiment would be in order, I’ve got a hypothesis, does anyone care to fund the test? 
I don’t know enough physiology to know the answer. Is there a physiologist in the house?
A few quick points I thought I’d add…
First of all, there are a LOT of things that would kill you before crush depth would…
First, your mixture of gasses that you’re breathing might kill you, too much oxygen at depth and you get oxygen toxicity, too much nitrogen and you’re at risk for nitrogen narcosis (rapture of the deep) not to mention the fact that it’s COLD down there and helium in your mixture is VERY thermally conductive, which results in ‘helium chills’ and hypothermia.
Assuming all of those don’t get you, or assuming you have a Liquid Breathing Apparatus (the hypothetical name postulated by some 80s sci-fi, hereafter LBA) or something else that is a perfect oxygen interface with none of the drawbacks of exotic gas mixtures and that will protect you from crushing… there are other hazards to worry about.
Like High-pressure nervous syndrome, which we don’t know a lot about, because no one has ever died from it. one of the 4 above got them first. But it’s basically time spent at high pressure affecting your CNS.
In other words, in a lot of ways, crushing is the least of your worries
Well, the distortion due to the lack of pressure (when you go higher in the atmosphere, and eventually into space) or the vast increase in pressure (when you’re under a lot of water) is what’s going to damage you, right? And, if the pressure is even all over you, the compression of your liquids and solids is going to be the only distortion, isn’t it? I’m just assuming that most of the liquids and solids in your body compress about the same degree that water does, which may not be a valid assumption.
Well, 8000 feet might just be insufficient to damage them enough. Seems like it should be plenty, you’d think, … but evidently sperm whales can survive to that depth, and it wouldn’t surprise me if liquid-breathing mice could too.
I thought the “According to the movie” part referred only to the first sentence – about the deepest human dive test being 4800 feet. I don’t recall hearing anything at all about mouse tests in the movie or the book, so I’m pretty sure that isn’t from the movie. I’d like to hear where they got that information, though.
Well, according to this page about sperm whales, which seems to have made a good effort to weed out crap from truth, we don’t know exactly how deep cetaceans can go (sperm whales, at least) but they’ve been measured by sonar at depths of 2500 meters, which is … let’s see … about 238 atmospheres. If red blood cells can withstand that, I’m curious about when they stop withstanding it.
Also, I thought it was mainly the creatures that had gas pockets in their bodies that died when brought to the surface from the ocean depths – no?
Sorry I wasn’t clear; I meant that no cetacean, even the sperm whale, is known to dive into the ocean trenches (even the shallower trenches are about 5,000 m or 16,000 ft deep… much less the Challenger Deep at about 10,000 m).
I don’t know where they got the information about the 8,000 ft mouse tests (wow, there’s a phrase that could be easily misconstrued into something hilarious 
). I would really like to find out. I agree that it’s unclear from the site and may not be from the movie, but I couldn’t find anything to substantiate it, and the site seems to have been made by some kind of student in a class project… it’s not unheard-of for students to make stuff up or to be credulous.
I really don’t know the physiological limits of pressure… I ran into a physiologist on Saturday and I asked her, but she didn’t know either (it wasn’t really her field of expertise though). I have a physiology book in storage, and I seem to recall a section on diving physiology in it, perhaps I’ll be able to take a look sometime.
Actually, most really deep sea critters don’t have gas pockets. There’s a high rate of death in attempted samples of invertebrates and IIRC, even many deep sea fish lack swim/gas bladders. See Bond, C. E. 1996. The Biology of Fishes, 2nd ed., page 284.
Here’s a bit from Bond’s book you may find interesting, relevant to those deep-sea fishes that do have gas bladders: (p. 287)
There are also apparently a variety of special ways to get gas in and out of bladders for those fish that do large vertical movements in the ocean.
Re: Bathyscape escape stunt would be/is impossible at its design depth. You wouldn’t be ablt to push the hatch outward against the pressure. (Surely you should have know that.)
Underwater exploration is now accomplished with sophisticated submersibles (remote controlled submarines). Expensive if loast but no one dies.
BTW: The Moho project and the space elevator just may get funded in time to set the base of the space elevator in the mohole.
I dug out my old physiology text and found this passage:
From: Schmidt-Nielsen, K. 1990. Animal Physiology. London: Cambridge University Press. pg. 192.
The reference to MacDonald is: MacDonald, A.G. 1975. Physiological Aspects of Deep Sea Biology. London: Cambridge University Press. 450 pp.
I don’t have the MacDonald text, but it seems to be the one that might elucidate what was known about high pressure at the time. It’s also very old, and quite possibly out of date.