In the book, World War Z, there are large herds of zombies walking across the floors of the oceans. How is that possible with that extreme pressure and the plethora of creepy-crawly ocean dwelling life eating away at them? The human body would literally come apart at that depth. And before someone says, "Decomposition slows down dramatically in that cold darkness", I would agree, but only if left completely still and protected from sea-life. The pressure would make the cells of the body literally fall apart if slight disturbed...let alone walking around while millions of ocean-going creatures are eating away at them.
The human body is highly compressible. This is because it is mostly liquids and solids, which will not deform under uniform pressure. Liquids and solids push back in response to pressure, and are for the most part "incompressible".
Q: Can you compress a liquid (water)?
The answer is yes, You can compress water, or almost any material. However, it requires a great deal of pressure to accomplish a little compression. For that reason, liquids and solids are sometimes referred to as being incompressible. The water at the bottom of the ocean is compressed by the weight of the water above it all the way to the surface, and is more dense than the water at the surface.
A calculation will show that this amount is very small indeed.
The density of water at 10,000 feet is only 1.3% higher than the density at the surface as a result of the pressure alone. When you take temperature and salinity into account, it is around 2.3% higher.
The pressure of the ocean wouldn't crush the flesh. It's the result of pressure on the gasses in the body that cause problems and would be fatal. Zombies don't have the same physiology, and so presumably don't suffer from these same problems.
Q: What would happen to an unprotected person at the bottom of the ocean or in outer space?
The water pressure is very high. The pressure from the water would push in on the person’s body, causing any space that’s filled with air to collapse. (The air would be compressed.) So, the lungs would collapse. At the same time, the pressure from the water would push water into the mouth, filling the lungs back up again with water instead of air. But if there’s no air-filled space to be pushed into, the body would not be crushed. (Part of the problem with the old pressure suits that deep-sea divers used to use was that if they depressurized, the soft part of the suit and the entire body would be crushed into the rigid helmet. This is one of the big reasons that divers don’t use suits like this anymore.)
Brooks' zombie lore says that animals instinctively avoid zombies. Although that claim is mainly backed up with "evidence" from land-dwelling animals, it is reasonable to assume it applies in the same fashion for sea-dwelling animals. These would simply not be interested in being anywhere near a zombie.
Further, regarding the pressure of the sea. I don't exactly remember the exact statement made as to the extent of the covered underwater area, but it's perfectly plausible that shallower seas can be roamed freely by the undead while those that walk to far out are crushed or at least incapacitated.
Maybe their legs and arms break before their skull does, so their brain remains intact, albeit immobile. Remember, a Brooks zombie doesn't have any use for lungs, heart, stomach or really anything below the skull (except for legs and arms for locomotion).
As James and Michael alluded to, there are some incorrect assumptions in your question.
The limiting factor in deep dives isn't the threat of being crushed by the pressure, it's the indirect results of pressure, e.g.
- Decompression time: a major limiting factor, as the deeper you go, the longer it takes to decompress, and the more dangerous it is if an emergency occurs and you need to rapidly ascend.
- Oxygen toxicity: at high pressures, oxygen becomes toxic, thus divers have to use trimix, heliox, or similar breathing gases, but those too have their limitations. Gas mixes that contain nitrogen can't be used at depth for long because of nitrogen narcosis, so it might be substituted with something else, like helium. But it seems that most gases are toxic at high enough concentrations/pressures.
- Oxygen supply: because of the time it takes to ascend/descend, deep dives require a lot of oxygen, and usually this is dealt with by having support divers plant oxygen at various depths for the primary dive team. Rebreathers remove carbon dioxide and recycle the oxygen from the diver's breath, significantly extending one's oxygen supply. But unforeseen problems can still occur, such as equipment failure, and once you run out of clean air, you're done.
So far, we don't yet know the hard limit for pressures on the human body, but it's easily above 33 atmospheres. The real limiting factor is that the margin for error at extreme depths is so low that even the highest rated technical divers in the world take their lives into their hands each time they try a record-breaking dive. For example, Dave Shaw has been considered one of the greatest technical divers in the world. Yet he died during a body recovery mission from a carbon-dioxide blackout because the skeleton of the dead diver got a line tangled up, and the physical exertion needed to untangle the lines exceeded his ability to exhale carbon dioxide (because he overfilled his rebreather).
Then there are other yet to be solved medical problems related to deep diving, such as aseptic bone necrosis (a form of avascular necrosis) due to the circulatory failure in the fine capillaries within our bones. This won't kill you immediately, but you're definitely risking life and limb by going to those pressures.
Simulated dives using decompression chambers have reached depths of 700+ meters, so at least short exposure to these pressures are survivable.
Zombies presumably don't need a working circulatory system, or oxygen, and they're already mentally impaired, so the effects of nitrogen narcosis and the likes won't be too much of an issue for them. If they slowly walk to their target depths, they should be fine from a pressure-resistance perspective, and staying mobile should keep most deep sea scavengers away (their size will keep away most predators, and deep sea creatures tend to have very conservative metabolisms because of the lower energy density in that ecosystem).
However, how will they stay submerged? The human body is naturally buoyant, which is why divers need to maintain neutral buoyancy using weights. Zombies, if they decompose, will become positively buoyant, which will making it impossible for them to walk on the sea floor without a diving weight system.
Also, if they manage to put on diving weights but then walk into ultra-deep parts of the sea floor, like the Challenger Deep or the Marianas Trench, the walls may simply be too steep for them to ascend on the other side.
So these are the real problems as I see them.
Just from a pure crush point of view a human bone can take 19000lbs/in2. 1 bar(unit of atmospheric pressure equal to sea level) is roughly 14lbs/in2. Every 10m(33f) of water pressure increases by 1 bar. So for the pressure to crush bone it would have to be at a depth of roughly 8 miles(13km). The deepest part of the ocean is approximately 6.8miles(11km).
None of this accounts for soft tissue or tendon strength, and even at a depth of only 100 meters every movement the zombie made would have to oppose approximately 150lbs of pressure per square inch, but there is no place in earthly oceans deep enough to crush human bones.
Problems with pressure:
- crushing effect - occurs when something compresses at a greater rate than the surrounding substance, and the surrounding substance doesn't let any other substance to take the place of the inner substance being compressed, AND the surrounding medium lacks the elasticity to collapse to compensate for the diminished volume, AND the surrounding medium isn't strong enough to withstand the force of the pressure differential. (all three needs to happen for something to get crushed, ie a tin can).
- compression -
- a. as pressure increases the density of substances increases, thus causing things to work differently (seen best in gases, like air). a good example of this is when you go deep enough in the ocean the air ends up sinking (it becomes denser than the surrounding water). divers add air into their BCs to counter this effect (less buoyancy from air) at much shallower depths. another example is that the energy needed to breath (push and pull air out of your lungs) at a certain point would exceed the energy you get from the air you breath.
- b. as density changes, so does the solubility of the substances (this is why divers have to worry about how much nitrogen, and oxygen the absorb at different depths). as nitrogen absorbs into the blood stream (into the blood plasma) the blood becomes even thicker and denser, this is due to the nitrogen compressing down to fit more into the same space (blood plasma being mostly water compresses at a much slower rate). normally this only causes a slight drunk effect, but eventually it would impair blood flow. as the pressure decreases the nitrogen would be unable to fit into the blood plasma and would come out at a rate based on the rate of decreasing pressure. normally the respiratory process expels excess gas from blood at a slow rate (zombies don't breath so it would have to happen at the rate of basic diffusion through materials which is much slower). exceeding this rate results in an effect similar to the bubbles in soda, which is much worse when it comes out of the little capillaries in ones tissues (the tissues would be ripped apart from the inside out). next issue we have is that our bodies operate off of chemical processes, one well known issue that occurs due to amounts absorbed into ones body is called oxygen toxicity. the exact process isn't too well known, but its effects are the damaging of nerve tissue (like the brain, which is most relevant to the discussion of zombies).
- c. as stuff compresses they heat up, as they uncompress the cool down. this is rarely an issue with in water compression as the rate of compression is usually low enough for the water to absorb the heat caused by compression. in air this is an issue though (so a leak in a space ship is a big deal if it is fast enough will freeze a zombie, or if you compress a zombie in a pressure chamber too fast it will fry). though this is less of an issue than crushing in water.
Crushing is most often used by science fiction writers due to the large numbers and general lack of knowledge in the area, however the other major effect from pressure (compression) results in interesting questions:
Do zombies retain a respiratory system, despite not needing one? if the don't then any zombie reaching any real depth for any length of time would be unable to return to the surface in any sort of time frame to pose a threat (and a hoard returning would be so spaced out as to no longer be a hoard). if they do retain one then they still would need considerable time to return and being mindless would need to need a shallow enough egress to keep from breaking apart. the third possibility is that there is some special mechanism to allow the zombies to deal with the fizzing issue.
Does killing the brain kill the zombie, or does it just need to be deformed? if the brain needs to live, then any mechanism that gives them oxygen would kill them at depth, however the human body lacks enough air capacity to cause oxygen toxicity based on the air volume at the surface (at 33 feet or 2 atmospheres a lung full of air has twice as much air than at the surface, and at 2 atmospheres pure oxygen will begin killing the brain). as zombies are unlikely to be outfitted to kill themselves (scuba gear at 264 feet or 8 atmospheres is a shoe in for brain death), but answering that question would allow one to formulate methods of killing zombies that don't rely on kinetic force (and over coming the protection of the skull).
So zombies on the ocean floor would be entirely possible, however they most likely would pose no threat to surface dwellers, and may even be an effective way to diminish their numbers (by them breaking apart as they ascend).